diff --git a/xkb/xkbtext.c b/xkb/xkbtext.c
index d2a2567fc..00a26c576 100644
--- a/xkb/xkbtext.c
+++ b/xkb/xkbtext.c
@@ -870,6 +870,7 @@ CopyISOLockArgs(XkbDescPtr xkb, XkbAction *action, char *buf, int *sz)
     XkbISOAction *act;
     char tbuf[64];
 
+    memset(tbuf, 0, sizeof(tbuf));
     act = &action->iso;
     if (act->flags & XkbSA_ISODfltIsGroup) {
         TryCopyStr(tbuf, "group=", sz);