diff --git a/config.c b/config.c
index 2ffd39c22006beaf8c4b0941d82183dd3b756955..04286f7717645c6812c7236a282c34f5ad353eb7 100644
--- a/config.c
+++ b/config.c
@@ -1668,6 +1668,8 @@ static int do_git_config_sequence(const struct config_options *opts,
 
 	if (opts->commondir)
 		repo_config = mkpathdup("%s/config", opts->commondir);
+	else if (opts->git_dir)
+		BUG("git_dir without commondir");
 	else
 		repo_config = NULL;