diff --git a/builtin/describe.c b/builtin/describe.c
index f7bea3c8c5..72b2e1162c 100644
--- a/builtin/describe.c
+++ b/builtin/describe.c
@@ -507,8 +507,10 @@ static void process_object(struct object *obj, const char *path, void *data)
 
 	if (oideq(pcd->looking_for, &obj->oid) && !pcd->dst->len) {
 		reset_revision_walk();
-		describe_commit(&pcd->current_commit, pcd->dst);
-		strbuf_addf(pcd->dst, ":%s", path);
+		if (!is_null_oid(&pcd->current_commit)) {
+			describe_commit(&pcd->current_commit, pcd->dst);
+			strbuf_addf(pcd->dst, ":%s", path);
+		}
 		free_commit_list(pcd->revs->commits);
 		pcd->revs->commits = NULL;
 	}
diff --git a/t/t6120-describe.sh b/t/t6120-describe.sh
index feec57bcbc..2c70cc561a 100755
--- a/t/t6120-describe.sh
+++ b/t/t6120-describe.sh
@@ -423,6 +423,22 @@ test_expect_success 'describe blob on an unborn branch' '
 	test_grep "cannot search .* on an unborn branch" actual
 '
 
+# This test creates a repository state that we generally try to disallow: HEAD
+# is pointing to an object that is not a commit. The ref update code forbids
+# non-commit writes directly to HEAD or to any branch in refs/heads/.  But we
+# can use the loophole of pointing HEAD to another non-branch ref (something we
+# should forbid, but don't for historical reasons).
+#
+# Do not take this test as an endorsement of the loophole! If we ever tighten
+# it, it is reasonable to just drop this test entirely.
+test_expect_success 'describe blob on a non-commit HEAD' '
+	oldbranch=$(git symbolic-ref HEAD) &&
+	test_when_finished "git symbolic-ref HEAD $oldbranch" &&
+	git symbolic-ref HEAD refs/tags/test-blob &&
+	test_must_fail git describe test-blob 2>actual &&
+	test_grep "blob .* not reachable from HEAD" actual
+'
+
 test_expect_success ULIMIT_STACK_SIZE 'name-rev works in a deep repo' '
 	i=1 &&
 	while test $i -lt 8000