diff --git a/sys/winnt/Makefile.msc b/sys/winnt/Makefile.msc
index 2a27bc1b5..e5f479925 100644
--- a/sys/winnt/Makefile.msc
+++ b/sys/winnt/Makefile.msc
@@ -251,10 +251,10 @@ CROSSCOMPILE = -DCROSSCOMPILE
 OPTIONS_AT_RUNTIME=Y
 HOST=_host
 !ELSE
-CROSSCOMPILE_TARGET =
-CROSSCOMPILE_HOST =
-CROSSCOMPILE =
-HOST=
+!UNDEF CROSSCOMPILE_TARGET
+!UNDEF CROSSCOMPILE_HOST
+!UNDEF CROSSCOMPILE
+!UNDEF HOST
 !ENDIF
 
 #
@@ -623,8 +623,8 @@ CL_RECENT=-sdl
 !IF "$(ADD_CURSES)" == "Y"
 CURSESDEF=-D"CURSES_GRAPHICS" -D"CURSES_BRIEF_INCLUDE" -DCHTYPE_32
 !ELSE
-CURSDEF=
-CURSESLIB=
+!UNDEF CURSDEF=
+!UNDEF CURSESLIB=
 !ENDIF
 
 ccommon= -c -nologo -D"_CONSOLE" -D"_CRT_NONSTDC_NO_DEPRECATE" -D"_CRT_SECURE_NO_DEPRECATE" \