diff --git a/build/GNUmakefile.libui b/build/GNUmakefile.libui
index cf61bc6a..393f054f 100644
--- a/build/GNUmakefile.libui
+++ b/build/GNUmakefile.libui
@@ -37,9 +37,9 @@ else
 	# make every symbol hidden by default except _UI_EXTERN ones
 	# TODO autogenerate a .def file?
 	CFLAGS += \
-		/D "_UI_EXTERN=__declspec(dllexport) extern"
+		-D "_UI_EXTERN=__declspec(dllexport) extern"
 	CXXFLAGS += \
-		/D "_UI_EXTERN=__declspec(dllexport) extern"
+		-D "_UI_EXTERN=__declspec(dllexport) extern"
 endif
 
 ifeq ($(USESSONAME),1)