diff --git a/lldb/packages/Python/lldbsuite/test/make/Makefile.rules b/lldb/packages/Python/lldbsuite/test/make/Makefile.rules
index 1ba3f843e87cf3..26e80df5e47314 100644
--- a/lldb/packages/Python/lldbsuite/test/make/Makefile.rules
+++ b/lldb/packages/Python/lldbsuite/test/make/Makefile.rules
@@ -46,6 +46,31 @@ LLDB_BASE_DIR := $(THIS_FILE_DIR)/../../../../../
 # according to variable values).
 .DEFAULT_GOAL := all
 
+#----------------------------------------------------------------------
+# If OS is not defined, use 'uname -s' to determine the OS name.
+#
+# GNUWin32 uname gives "windows32" or "server version windows32" while
+# some versions of MSYS uname return "MSYS_NT*", but most environments
+# standardize on "Windows_NT", so we'll make it consistent here.
+# When running tests from Visual Studio, the environment variable isn't
+# inherited all the way down to the process spawned for make.
+#----------------------------------------------------------------------
+ifeq "$(HOST_OS)" ""
+  HOST_OS := $(shell uname -s)
+endif
+
+ifneq (,$(findstring windows32,$(HOST_OS)))
+	HOST_OS := Windows_NT
+endif
+
+ifneq (,$(findstring MSYS_NT,$(HOST_OS)))
+	HOST_OS := Windows_NT
+endif
+
+ifeq "$(OS)" ""
+	OS := $(HOST_OS)
+endif
+
 #----------------------------------------------------------------------
 # If OS is Windows, force SHELL to be cmd
 #
@@ -596,7 +621,6 @@ ifeq "$(MAKE_DWP)" "YES"
 endif
 endif
 
-
 #----------------------------------------------------------------------
 # Make the dylib
 #----------------------------------------------------------------------