diff --git a/src/os_unix.c b/src/os_unix.c
index b49afc3..6209605 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -1960,9 +1960,12 @@
     return retval;
 }
 
-/* Are Xutf8 functions available?  Avoid error from old compilers. */
+/* Xutf8 functions are not avaialble on older systems. Note that on some
+ * systems X_HAVE_UTF8_STRING may be defined in a header file but
+ * Xutf8SetWMProperties() is not in the X11 library.  Configure checks for
+ * that and defines HAVE_XUTF8SETWMPROPERTIES. */
 #if defined(X_HAVE_UTF8_STRING) && defined(FEAT_MBYTE)
-# if X_HAVE_UTF8_STRING
+# if X_HAVE_UTF8_STRING && HAVE_XUTF8SETWMPROPERTIES
 #  define USE_UTF8_STRING
 # endif
 #endif