diff --git a/pack-write.c b/pack-write.c
index 9ccf80419..33293ce2a 100644
--- a/pack-write.c
+++ b/pack-write.c
@@ -288,13 +288,12 @@ char *index_pack_lockfile(int ip_out)
 	 * case, we need it to remove the corresponding .keep file
 	 * later on.  If we don't get that then tough luck with it.
 	 */
-	if (read_in_full(ip_out, packname, 46) == 46 && packname[45] == '\n' &&
-	    memcmp(packname, "keep\t", 5) == 0) {
-		char path[PATH_MAX];
+	if (read_in_full(ip_out, packname, 46) == 46 && packname[45] == '\n') {
+		const char *name;
 		packname[45] = 0;
-		snprintf(path, sizeof(path), "%s/pack/pack-%s.keep",
-			 get_object_directory(), packname + 5);
-		return xstrdup(path);
+		if (skip_prefix(packname, "keep\t", &name))
+			return xstrfmt("%s/pack/pack-%s.keep",
+				       get_object_directory(), name);
 	}
 	return NULL;
 }