diff --git a/conans/client/subsystems.py b/conans/client/subsystems.py
index 3a1f23f47e8..25511f270f7 100644
--- a/conans/client/subsystems.py
+++ b/conans/client/subsystems.py
@@ -104,6 +104,8 @@ def deduce_subsystem(conanfile, scope):
     if scope.startswith("build"):
         if hasattr(conanfile, "settings_build"):
             the_os = conanfile.settings_build.get_safe("os")
+            if the_os is None:
+                raise ConanException("The 'build' profile must have a 'os' declared")
             subsystem = conanfile.settings_build.get_safe("os.subsystem")
         else:
             the_os = platform.system()  # FIXME: Temporary fallback until 2.0
diff --git a/conans/test/integration/build_requires/profile_build_requires_test.py b/conans/test/integration/build_requires/profile_build_requires_test.py
index de19da8a9c5..134c6b61cf9 100644
--- a/conans/test/integration/build_requires/profile_build_requires_test.py
+++ b/conans/test/integration/build_requires/profile_build_requires_test.py
@@ -197,6 +197,7 @@ def test_consumer_patterns_loop_error():
     client = TestClient()
 
     profile_patterns = textwrap.dedent("""
+        include(default)
         [tool_requires]
         tool1/1.0
         tool2/1.0
@@ -208,12 +209,12 @@ def test_consumer_patterns_loop_error():
 
     client.run("export tool1 --name=tool1 --version=1.0")
     client.run("export tool2 --name=tool2 --version=1.0")
-    with pytest.raises(Exception) as e:
-        client.run("install consumer --build=missing -pr:b=profile.txt -pr:h=profile.txt")
-    assert "graph loop" in str(e.value)
+    client.run("install consumer --build=missing -pr:b=profile.txt -pr:h=profile.txt", assert_error=True)
+    assert "graph loop" in client.out
 
     # we can fix it with the negation
     profile_patterns = textwrap.dedent("""
+        include(default)
         [tool_requires]
         tool1/1.0
         !tool1*:tool2/1.0
diff --git a/conans/test/integration/command/create_test.py b/conans/test/integration/command/create_test.py
index 6921d78aa64..25fdfffe8cd 100644
--- a/conans/test/integration/command/create_test.py
+++ b/conans/test/integration/command/create_test.py
@@ -15,8 +15,6 @@ class CreateTest(unittest.TestCase):
 
     def test_dependencies_order_matches_requires(self):
         client = TestClient()
-        save(client.cache.default_profile_path, "")
-        save(client.cache.settings_path, "build_type: [Release, Debug]\narch: [x86]")
         client.save({"conanfile.py": GenConanfile()})
         client.run("create . --name=pkga --version=0.1 --user=user --channel=testing")
         client.save({"conanfile.py": GenConanfile()})
diff --git a/conans/test/integration/command/upload/syncronize_test.py b/conans/test/integration/command/upload/syncronize_test.py
index 975fa5a5112..fe3bdbd1dc3 100644
--- a/conans/test/integration/command/upload/syncronize_test.py
+++ b/conans/test/integration/command/upload/syncronize_test.py
@@ -15,7 +15,6 @@ class SynchronizeTest(unittest.TestCase):
 
     def test_upload(self):
         client = TestClient(servers={"default": TestServer()}, inputs=["admin", "password"])
-        save(client.cache.default_profile_path, "")
         ref = RecipeReference.loads("hello/0.1@lasote/stable")
         files = {"conanfile.py": GenConanfile("hello", "0.1").with_exports("*"),
                  "to_be_deleted.txt": "delete me",
diff --git a/conans/test/integration/command/upload/upload_complete_test.py b/conans/test/integration/command/upload/upload_complete_test.py
index e4dd44ff3fd..b585ac577cd 100644
--- a/conans/test/integration/command/upload/upload_complete_test.py
+++ b/conans/test/integration/command/upload/upload_complete_test.py
@@ -97,7 +97,6 @@ def _get_client(self, requester=None):
         servers["default"] = self.test_server
         test_client = TestClient(servers=servers, inputs=["lasote", "mypass"],
                                  requester_class=requester)
-        save(test_client.cache.default_profile_path, "")
         return test_client
 
     def test_upload_error(self):
diff --git a/conans/test/integration/configuration/default_profile_test.py b/conans/test/integration/configuration/default_profile_test.py
index 11f967f94aa..9546ed20635 100644
--- a/conans/test/integration/configuration/default_profile_test.py
+++ b/conans/test/integration/configuration/default_profile_test.py
@@ -19,7 +19,6 @@ def test_conanfile_txt_incomplete_profile(self):
         conanfile = GenConanfile()
 
         client = TestClient()
-        save(client.cache.default_profile_path, "")
 
         client.save({CONANFILE: conanfile})
         client.run("create . --name=pkg --version=0.1 --user=lasote --channel=stable")
@@ -165,7 +164,7 @@ def build(self):
 
         # Test with the 'default' profile
         env_variable = "env_variable=profile_default"
-        save(client.cache.default_profile_path, "[buildenv]\n" + env_variable)
+        save(client.cache.default_profile_path, "[settings]\nos=Windows\n[buildenv]\n" + env_variable)
         client.run("create . --name=name --version=version --user=user --channel=channel")
         self.assertIn(">>> " + env_variable, client.out)
 
@@ -173,7 +172,7 @@ def build(self):
         tmp = temp_folder()
         env_variable = "env_variable=profile_environment"
         default_profile_path = os.path.join(tmp, 'env_profile')
-        save(default_profile_path, "[buildenv]\n" + env_variable)
+        save(default_profile_path, "[settings]\nos=Windows\n[buildenv]\n" + env_variable)
         with environment_update({'CONAN_DEFAULT_PROFILE': default_profile_path}):
             client.run("create . --name=name --version=version --user=user --channel=channel")
             self.assertIn(">>> " + env_variable, client.out)
@@ -184,7 +183,7 @@ def build(self):
         self.assertFalse(os.path.isabs(rel_path))
         default_profile_path = os.path.join(client.cache_folder,
                                             PROFILES_FOLDER, rel_path)
-        save(default_profile_path, "[buildenv]\n" + env_variable)
+        save(default_profile_path, "[settings]\nos=Windows\n[buildenv]\n" + env_variable)
         with environment_update({'CONAN_DEFAULT_PROFILE': rel_path}):
             client.run("create . --name=name --version=version --user=user --channel=channel")
             self.assertIn(">>> " + env_variable, client.out)
diff --git a/conans/test/integration/graph/core/test_version_ranges.py b/conans/test/integration/graph/core/test_version_ranges.py
index 929ec472c04..bb1fc8186b6 100644
--- a/conans/test/integration/graph/core/test_version_ranges.py
+++ b/conans/test/integration/graph/core/test_version_ranges.py
@@ -368,8 +368,6 @@ def test_mixed_user_channel():
 
 def test_remote_version_ranges():
     t = TestClient(default_server_user=True)
-    save(t.cache.default_profile_path, "")
-    save(t.cache.settings_path, "")
     t.save({"conanfile.py": GenConanfile()})
     for v in ["0.1", "0.2", "0.3", "1.1", "1.1.2", "1.2.1", "2.1", "2.2.1"]:
         t.run(f"create . --name=dep --version={v}")
@@ -406,8 +404,6 @@ def test_different_user_channel_resolved_correctly():
     servers = OrderedDict([("server1", server1), ("server2", server2)])
 
     client = TestClient(servers=servers, inputs=2*["admin", "password"])
-    save(client.cache.default_profile_path, "")
-    save(client.cache.settings_path, "")
     client.save({"conanfile.py": GenConanfile()})
     client.run("create . --name=lib --version=1.0 --user=conan --channel=stable")
     client.run("create . --name=lib --version=1.0 --user=conan --channel=testing")
diff --git a/conans/test/integration/package_id/test_cache_compatibles.py b/conans/test/integration/package_id/test_cache_compatibles.py
index 907de4847cb..fc8b4566a32 100644
--- a/conans/test/integration/package_id/test_cache_compatibles.py
+++ b/conans/test/integration/package_id/test_cache_compatibles.py
@@ -143,7 +143,7 @@ class Pkg(ConanFile):
                 package_type = "application"
                 settings = "os", "arch", "compiler", "build_type"
             """)
-        c.save({"conanfile.py": conanfile})
+        c.save({"conanfile.py": conanfile, "profile_build": "[settings]\nos=Windows\narch=x86_64"})
         os_ = "Windows"
         build_type = "Release"
         arch = "x86_64"
@@ -155,9 +155,9 @@ class Pkg(ConanFile):
         c.run(f"create . -s os={os_} -s arch={arch} -s build_type={build_type} "
               f"-s compiler={compiler} "
               f"-s compiler.version={version} -s compiler.cppstd={cppstd} "
-              f"-s compiler.runtime={runtime}")
+              f"-s compiler.runtime={runtime} -pr:b=profile_build")
         package_id = c.created_package_id("app/1.0")
-        c.run(f"install --requires=app/1.0@ -s os={os_} -s arch={arch}")
+        c.run(f"install --requires=app/1.0@ -s os={os_} -s arch={arch} -pr:b=profile_build")
         assert "app/1.0: Main binary package 'e340edd75790e7156c595edebd3d98b10a2e091e' missing."\
                f"Using compatible package '{package_id}'"
 
@@ -178,7 +178,7 @@ def package_id(self):
                     except:
                         pass
             """)
-        c.save({"conanfile.py": conanfile})
+        c.save({"conanfile.py": conanfile, "profile_build": "[settings]\nos=Windows\narch=x86_64"})
         os_ = "Windows"
         build_type = "Release"
         arch = "x86_64"
@@ -189,16 +189,16 @@ def package_id(self):
         c.run(f"create . -s os={os_} -s arch={arch} -s build_type={build_type} "
               f"-s compiler={compiler} "
               f"-s compiler.version={version} -s compiler.cppstd={cppstd} "
-              f"-s compiler.runtime={runtime}")
+              f"-s compiler.runtime={runtime} -pr:b=profile_build")
         package_id1 = c.created_package_id("app/1.0")
         c.run(f"create . -s os={os_} -s arch={arch} -s build_type={build_type} "
               f"-s compiler={compiler} "
               f"-s compiler.version={version} -s compiler.cppstd=17 "
-              f"-s compiler.runtime={runtime}")
+              f"-s compiler.runtime={runtime} -pr:b=profile_build")
         package_id2 = c.created_package_id("app/1.0")
         assert package_id1 == package_id2  # It does not depend on 'compiler.cppstd'
 
-        c.run(f"install --requires=app/1.0@ -s os={os_} -s arch={arch}")
+        c.run(f"install --requires=app/1.0@ -s os={os_} -s arch={arch} -pr:b=profile_build")
         assert "app/1.0: Main binary package 'e340edd75790e7156c595edebd3d98b10a2e091e' missing."\
                f"Using compatible package '{package_id1}'"
 
@@ -215,7 +215,7 @@ class Pkg(ConanFile):
                 default_options = {"shared": False}
                 settings = "os", "arch", "compiler", "build_type"
             """)
-        c.save({"conanfile.py": conanfile})
+        c.save({"conanfile.py": conanfile, "profile_build": "[settings]\nos=Windows\narch=x86_64"})
         os_ = "Windows"
         build_type = "Release"
         arch = "x86_64"
@@ -226,14 +226,14 @@ class Pkg(ConanFile):
         c.run(f"create . -s os={os_} -s arch={arch} -s build_type={build_type} "
               f"-s compiler={compiler} "
               f"-s compiler.version={version} -s compiler.cppstd={cppstd} "
-              f"-s compiler.runtime={runtime}")
+              f"-s compiler.runtime={runtime} -pr:b=profile_build")
         package_id1 = c.created_package_id("mylib/1.0")
 
         # Try to install with cppstd 14, it will find cppstd 17 as compatible
         c.run(f"install --requires=mylib/1.0@ -s os={os_} -s arch={arch} -s build_type={build_type} "
               f"-s compiler={compiler} "
               f"-s compiler.version={version} -s compiler.cppstd=14 "
-              f"-s compiler.runtime={runtime}")
+              f"-s compiler.runtime={runtime} -pr:b=profile_build")
         assert "mylib/1.0: Main binary package 'e340edd75790e7156c595edebd3d98b10a2e091e' missing."\
                f"Using compatible package '{package_id1}'"
 
diff --git a/conans/test/integration/toolchains/cmake/test_cmake.py b/conans/test/integration/toolchains/cmake/test_cmake.py
index fc3cef0a501..86ae90d270d 100644
--- a/conans/test/integration/toolchains/cmake/test_cmake.py
+++ b/conans/test/integration/toolchains/cmake/test_cmake.py
@@ -1,5 +1,4 @@
 import textwrap
-
 from conans.test.utils.tools import TestClient
 
 
diff --git a/conans/test/integration/toolchains/env/test_buildenv.py b/conans/test/integration/toolchains/env/test_buildenv.py
new file mode 100644
index 00000000000..bc6a4047094
--- /dev/null
+++ b/conans/test/integration/toolchains/env/test_buildenv.py
@@ -0,0 +1,39 @@
+import platform
+import textwrap
+
+import pytest
+
+from conans.test.utils.tools import TestClient
+
+
+@pytest.mark.skipif(platform.system() != "Windows", reason="Fails on windows")
+def test_crossbuild_windows_incomplete():
+    client = TestClient()
+    conanfile = textwrap.dedent("""
+        from conan import ConanFile
+        from conan.tools.cmake import CMake
+        class Pkg(ConanFile):
+            name = "pkg"
+            version = "0.1"
+            settings = "os", "compiler", "build_type", "arch"
+            generators = "CMakeToolchain"
+            def build(self):
+                self.run("dir c:")
+            """)
+    build_profile = textwrap.dedent("""
+        [settings]
+        arch=x86_64
+    """)
+    client.save({"conanfile.py": conanfile, "build_profile": build_profile})
+    client.run("create . -pr:b=build_profile", assert_error=True)
+    assert "'.' is not recognized as an internal or external command" not in client.out
+    assert "ERROR: The 'build' profile must have a 'os' declared" in client.out
+
+    build_profile = textwrap.dedent("""
+            [settings]
+            os=Windows
+            arch=x86_64
+        """)
+    client.save({"conanfile.py": conanfile, "build_profile": build_profile})
+    client.run("create . -pr:b=build_profile")
+    assert "<DIR>          ." in client.out
diff --git a/conans/test/unittests/client/profile_loader/profile_loader_test.py b/conans/test/unittests/client/profile_loader/profile_loader_test.py
index dfdbf72c7ea..28c9316940f 100644
--- a/conans/test/unittests/client/profile_loader/profile_loader_test.py
+++ b/conans/test/unittests/client/profile_loader/profile_loader_test.py
@@ -6,7 +6,7 @@
 from conans.client.profile_loader import _ProfileParser, ProfileLoader
 from conans.errors import ConanException
 from conans.model.recipe_ref import RecipeReference
-from conans.test.utils.mocks import ConanFileMock
+from conans.test.utils.mocks import ConanFileMock, MockSettings
 from conans.test.utils.test_files import temp_folder
 from conans.util.files import save
 
@@ -212,11 +212,15 @@ def test_profile_buildenv():
     buildenv = profile.buildenv
     env = buildenv.get_profile_env(None)
     conanfile = ConanFileMock()
+    conanfile.settings_build = MockSettings({"os": "Linux", "arch": "x86_64"})
     env_vars = env.vars(conanfile)
     assert env_vars.get("MyVar1") == "My Value; 11 MyValue12"
-    # Mock is never Windows path
     assert env_vars.get("MyPath1") == "/some/path11:/other path/path12"
 
+    conanfile.settings_build = MockSettings({"os": "Windows", "arch": "x86_64"})
+    env_vars = env.vars(conanfile)
+    assert env_vars.get("MyPath1") == "/some/path11;/other path/path12"
+
 
 @pytest.mark.parametrize("conf_name", [
     "core.doesnotexist:never",
diff --git a/conans/test/unittests/client/toolchain/autotools/autotools_toolchain_test.py b/conans/test/unittests/client/toolchain/autotools/autotools_toolchain_test.py
index c0eae5f87fe..c9cb62f6d26 100644
--- a/conans/test/unittests/client/toolchain/autotools/autotools_toolchain_test.py
+++ b/conans/test/unittests/client/toolchain/autotools/autotools_toolchain_test.py
@@ -135,6 +135,7 @@ def test_ndebug():
     conanfile = ConanFileMock()
     for bt in ['Release', 'RelWithDebInfo', 'MinSizeRel']:
         conanfile.settings = MockSettings({"build_type": bt})
+        conanfile.settings_build = MockSettings({"os": "Linux", "arch": "x86_64"})
         be = AutotoolsToolchain(conanfile)
         assert be.ndebug == "NDEBUG"
         env = be.vars()
@@ -166,8 +167,10 @@ def test_ndebug():
 def test_libcxx(config):
     compiler, libcxx, expected_flag = config
     conanfile = ConanFileMock()
+    the_os = "Linux" if compiler != "apple-clang" else "Macos"
     conanfile.settings = MockSettings(
-        {"build_type": "Release",
+        {"os": the_os,
+         "build_type": "Release",
          "arch": "x86",
          "compiler": compiler,
          "compiler.libcxx": libcxx,
@@ -184,7 +187,8 @@ def test_libcxx(config):
 def test_cxx11_abi_define():
     conanfile = ConanFileMock()
     conanfile.settings = MockSettings(
-        {"build_type": "Release",
+        {"os": "Linux",
+         "build_type": "Release",
          "arch": "x86",
          "compiler": "gcc",
          "compiler.libcxx": "libstdc++",
@@ -197,7 +201,8 @@ def test_cxx11_abi_define():
     assert "-D_GLIBCXX_USE_CXX11_ABI=0" in env["CPPFLAGS"]
 
     conanfile.settings = MockSettings(
-        {"build_type": "Release",
+        {"os": "Linux",
+         "build_type": "Release",
          "arch": "x86",
          "compiler": "gcc",
          "compiler.libcxx": "libstdc++11",
diff --git a/conans/test/unittests/tools/env/test_env.py b/conans/test/unittests/tools/env/test_env.py
index 362ac9653ee..b24ef4da6b0 100644
--- a/conans/test/unittests/tools/env/test_env.py
+++ b/conans/test/unittests/tools/env/test_env.py
@@ -188,7 +188,9 @@ def env():
     env.append("MYVAR1", "MyValue1B")
     env.define("MyVar2", "MyNewValue2")
 
-    env = env.vars(ConanFileMock())
+    conanfile = ConanFileMock()
+    conanfile.settings_build = MockSettings({"os": "Windows", "arch": "x86_64"})
+    env = env.vars(conanfile)
     env._subsystem = WINDOWS
 
     return env
diff --git a/conans/test/utils/mocks.py b/conans/test/utils/mocks.py
index b8b3deede32..0a141f25117 100644
--- a/conans/test/utils/mocks.py
+++ b/conans/test/utils/mocks.py
@@ -103,14 +103,14 @@ def run(self, *args, **kwargs):
 
 class ConanFileMock(ConanFile):
 
-    def __init__(self, shared=None, ):
+    def __init__(self, shared=None):
         self.display_name = ""
         self._conan_node = None
         self.command = None
         self._commands = []
         self.path = None
         self.settings = None
-        self.settings_build = MockSettings({})
+        self.settings_build = MockSettings({"os": "Linux", "arch": "x86_64"})
         self.options = Options()
         if shared is not None:
             self.options = namedtuple("options", "shared")(shared)