diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 8ee08ff2fd..0ca986f721 100755
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -475,6 +475,9 @@ do
 
 		require_clean_work_tree
 
+		UPSTREAM=$(git rev-parse --verify "$1") || die "Invalid base"
+		test -z "$ONTO" && ONTO=$UPSTREAM
+
 		if test ! -z "$2"
 		then
 			output git show-ref --verify --quiet "refs/heads/$2" ||
@@ -484,12 +487,8 @@ do
 		fi
 
 		HEAD=$(git rev-parse --verify HEAD) || die "No HEAD?"
-		UPSTREAM=$(git rev-parse --verify "$1") || die "Invalid base"
-
 		mkdir "$DOTEST" || die "Could not create temporary $DOTEST"
 
-		test -z "$ONTO" && ONTO=$UPSTREAM
-
 		: > "$DOTEST"/interactive || die "Could not mark as interactive"
 		git symbolic-ref HEAD > "$DOTEST"/head-name 2> /dev/null ||
 			echo "detached HEAD" > "$DOTEST"/head-name