diff --git a/src/mbyte.c b/src/mbyte.c
index 646235f..ccf23e8 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -5735,7 +5735,7 @@
 #endif
 
 #if defined(FEAT_GUI_X11) || defined(PROTO)
-# if defined(XtSpecificationRelease) && XtSpecificationRelease >= 6 && !defined(sun)
+# if defined(XtSpecificationRelease) && XtSpecificationRelease >= 6 && !defined(SUN_SYSTEM)
 #  define USE_X11R6_XIM
 # endif