--- a/Makefile.am
+++ b/Makefile.am
@@ -61,7 +61,7 @@ COMMON_TESTING_PROTOBUF_FLAGS = $(COMMON
 
 # The generated protobuf files don't compile with -Werror on win32 so we
 # disable fatal warnings on WIN32.
-if ! USING_WIN32
+if USING_WIN32
 if FATAL_WARNINGS
   COMMON_CXXFLAGS += -Werror
   COMMON_PROTOBUF_CXXFLAGS += -Werror -Wno-error=unused-parameter \
