if (merge_filter != NO_FILTER) {
                struct commit *filter;
                filter = lookup_commit_reference_gently(merge_filter_ref, 0);
+               if (!filter)
+                       die("object '%s' does not point to a commit",
+                           sha1_to_hex(merge_filter_ref));
+
                filter->object.flags |= UNINTERESTING;
                add_pending_object(&ref_list.revs,
                                   (struct object *) filter, "");