diff --git a/builtin/show-branch.c b/builtin/show-branch.c
index 408ce70307..092b59b0b3 100644
--- a/builtin/show-branch.c
+++ b/builtin/show-branch.c
@@ -743,6 +743,8 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 			fake_av[1] = NULL;
 			av = fake_av;
 			ac = 1;
+			if (!*av)
+				die("no branches given, and HEAD is not valid");
 		}
 		if (ac != 1)
 			die("--reflog option needs one branch name");