diff --git a/git-rebase.sh b/git-rebase.sh
index 15da926ce0..d6c52e6f90 100755
--- a/git-rebase.sh
+++ b/git-rebase.sh
@@ -464,7 +464,7 @@ case "$#" in
 		head_name="detached HEAD"
 		branch_name=HEAD ;# detached
 	fi
-	orig_head=$(git rev-parse --verify "${branch_name}^0") || exit
+	orig_head=$(git rev-parse --verify HEAD) || exit
 	;;
 *)
 	die "BUG: unexpected number of arguments left to parse"
diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
index 1de0ebda25..e6af021c10 100755
--- a/t/t3400-rebase.sh
+++ b/t/t3400-rebase.sh
@@ -101,7 +101,14 @@ test_expect_success 'HEAD was detached during rebase' '
 	test $(git rev-parse HEAD@{1}) != $(git rev-parse my-topic-branch@{1})
 '
 
+test_expect_success 'rebase from ambiguous branch name' '
+	git checkout -b topic side &&
+	git rebase master
+'
+
 test_expect_success 'rebase after merge master' '
+	git checkout --detach refs/tags/topic &&
+	git branch -D topic &&
 	git reset --hard topic &&
 	git merge master &&
 	git rebase master &&