diff --git a/xkb/xkbtext.c b/xkb/xkbtext.c
index 53b98c848..dd826b189 100644
--- a/xkb/xkbtext.c
+++ b/xkb/xkbtext.c
@@ -139,11 +139,11 @@ XkbVModIndexText(XkbDescPtr xkb, unsigned ndx, unsigned format)
 
     len = strlen(tmp) + 1;
     if (format == XkbCFile)
-        len += 4;
+        len += 5;
     rtrn = tbGetBuffer(len);
     if (format == XkbCFile) {
         strcpy(rtrn, "vmod_");
-        strncpy(&rtrn[5], tmp, len - 4);
+        strncpy(&rtrn[5], tmp, len - 5);
     }
     else
         strncpy(rtrn, tmp, len);