diff --git a/win64.mak b/win64.mak
index 87dda83..b28ad79 100755
--- a/win64.mak
+++ b/win64.mak
@@ -105,7 +105,7 @@ ADD_DEFINES = $(ADD_DEFINES) /Wp64
 !ENDIF
 
 !IF "$(PG_INC)" != ""
-INC_OPT = $(INC_OPT) /I "$(PG_INC)"
+INC_OPT = $(INC_OPT) /I "$(PG_INC)" /I "$(PG_INC)\internal"
 !ENDIF
 !IF "$(ADD_INC)" != ""
 INC_OPT = $(INC_OPT) /I "$(ADD_INC)"