diff --git a/test/posix-mock-test.cc b/test/posix-mock-test.cc
index 87ac3982..f79afd98 100644
--- a/test/posix-mock-test.cc
+++ b/test/posix-mock-test.cc
@@ -7,7 +7,9 @@
 
 // Disable bogus MSVC warnings.
 #ifdef _MSC_VER
-#  define _CRT_SECURE_NO_WARNINGS
+#  ifndef _CRT_SECURE_NO_WARNINGS
+#    define _CRT_SECURE_NO_WARNINGS
+#  endif
 #endif
 
 #include "posix-mock.h"