diff --git a/contrib/libreadline/vi_mode.c b/contrib/libreadline/vi_mode.c
index 5d146b3f7056..5eb176e10a62 100644
--- a/contrib/libreadline/vi_mode.c
+++ b/contrib/libreadline/vi_mode.c
@@ -1,3 +1,4 @@
+/* $FreeBSD$ */
 /* vi_mode.c -- A vi emulation mode for Bash.
    Derived from code written by Jeff Sparkes (jsparkes@bnr.ca).  */
 
@@ -680,7 +681,8 @@ _rl_vi_change_mbchar_case (count)
      int count;
 {
   wchar_t wc;
-  char mb[MB_LEN_MAX];
+  char mb[MB_LEN_MAX+1];
+  int mblen;
   mbstate_t ps;
 
   memset (&ps, 0, sizeof (mbstate_t));
@@ -703,7 +705,9 @@ _rl_vi_change_mbchar_case (count)
       /* Vi is kind of strange here. */
       if (wc)
 	{
-	  wctomb (mb, wc);
+	  mblen = wctomb (mb, wc);
+	  if (mblen >= 0)
+	    mb[mblen] = '\0';
 	  rl_begin_undo_group ();
 	  rl_delete (1, 0);
 	  rl_insert_text (mb);