diff --git a/Makefile.mingw b/Makefile.mingw
index 3b9827d1..40cf14b3 100644
--- a/Makefile.mingw
+++ b/Makefile.mingw
@@ -4,7 +4,7 @@ WINDRES = windres
 CXXFLAGS = -Os -D_MT -DWIN32 -D_WINDOWS -DWIN32_LEAN_AND_MEAN
 NEEDED_CXXFLAGS = -std=c++11
 BOOST_SUFFIX = -mt
-INCFLAGS += -I/usr/include/ -I/usr/local/include/
+INCFLAGS = -I/usr/include/ -I/usr/local/include/
 LDFLAGS = -Wl,-rpath,/usr/local/lib \
   -L/usr/local/lib