Index: modules/mappers/mod_rewrite.c
===================================================================
--- modules/mappers/mod_rewrite.c	(revision 917978)
+++ modules/mappers/mod_rewrite.c	(working copy)
@@ -3014,7 +3014,7 @@
     lockname = ap_server_root_relative(cmd->pool, a1);
 
     if (!lockname) {
-        return apr_pstrcat(cmd->pool, "Invalid RewriteLock path ", a1);
+        return apr_pstrcat(cmd->pool, "Invalid RewriteLock path ", a1, NULL);
     }
 
     return NULL;