diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index efcd875f91..5d70cc4286 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -41,6 +41,9 @@ __gitdir ()
 	if [ -z "${1-}" ]; then
 		if [ -n "${__git_dir-}" ]; then
 			echo "$__git_dir"
+		elif [ -n "${GIT_DIR-}" ]; then
+			test -d "${GIT_DIR-}" || return 1
+			echo "$GIT_DIR"
 		elif [ -d .git ]; then
 			echo .git
 		else
diff --git a/t/t9903-bash-prompt.sh b/t/t9903-bash-prompt.sh
index f8fb93f687..f17c1f8b85 100755
--- a/t/t9903-bash-prompt.sh
+++ b/t/t9903-bash-prompt.sh
@@ -85,7 +85,7 @@ test_expect_success 'gitdir - parent is a .git directory' '
 	test_cmp expected "$actual"
 '
 
-test_expect_failure 'gitdir - $GIT_DIR set while .git directory in cwd' '
+test_expect_success 'gitdir - $GIT_DIR set while .git directory in cwd' '
 	echo "$TRASH_DIRECTORY/otherrepo/.git" > expected &&
 	(
 		GIT_DIR="$TRASH_DIRECTORY/otherrepo/.git" &&
@@ -106,6 +106,14 @@ test_expect_success 'gitdir - $GIT_DIR set while .git directory in parent' '
 	test_cmp expected "$actual"
 '
 
+test_expect_success 'gitdir - non-existing $GIT_DIR' '
+	(
+		GIT_DIR="$TRASH_DIRECTORY/non-existing" &&
+		export GIT_DIR &&
+		test_must_fail __gitdir
+	)
+'
+
 test_expect_success 'gitdir - gitfile in cwd' '
 	echo "$TRASH_DIRECTORY/otherrepo/.git" > expected &&
 	echo "gitdir: $TRASH_DIRECTORY/otherrepo/.git" > subdir/.git &&