diff --git a/conans/client/tools/oss.py b/conans/client/tools/oss.py
index 2c7106e7079..4304b0baa00 100644
--- a/conans/client/tools/oss.py
+++ b/conans/client/tools/oss.py
@@ -24,6 +24,9 @@ def cpu_count(output=None):
     output = default_output(output, 'conans.client.tools.oss.cpu_count')
     try:
         env_cpu_count = os.getenv("CONAN_CPU_COUNT", None)
+        if env_cpu_count is not None and not env_cpu_count.isdigit():
+            raise ConanException("Invalid CONAN_CPU_COUNT value '%s', "
+                                 "please specify a positive integer" % env_cpu_count)
         return int(env_cpu_count) if env_cpu_count else multiprocessing.cpu_count()
     except NotImplementedError:
         output.warn("multiprocessing.cpu_count() not implemented. Defaulting to 1 cpu")
diff --git a/conans/test/unittests/util/tools_test.py b/conans/test/unittests/util/tools_test.py
index b3760fe2281..8d587c8ea9f 100644
--- a/conans/test/unittests/util/tools_test.py
+++ b/conans/test/unittests/util/tools_test.py
@@ -547,6 +547,9 @@ def cpu_count_test(self):
         self.assertGreaterEqual(cpus, 1)
         with tools.environment_append({"CONAN_CPU_COUNT": "34"}):
             self.assertEquals(tools.cpu_count(output=output), 34)
+        with tools.environment_append({"CONAN_CPU_COUNT": "null"}):
+            with self.assertRaisesRegexp(ConanException, "Invalid CONAN_CPU_COUNT value"):
+                tools.cpu_count(output=output)
 
     def get_env_unit_test(self):
         """