diff --git a/git-merge-one-file.sh b/git-merge-one-file.sh
index b86402afa5d079df8d1cf6eebb15e5727cc8a5de..7aeb96952f668baef2fd18383806e13a20415f19 100755
--- a/git-merge-one-file.sh
+++ b/git-merge-one-file.sh
@@ -22,6 +22,11 @@ LONG_USAGE="Usage: git merge-one-file $USAGE
 
 Blob ids and modes should be empty for missing files."
 
+SUBDIRECTORY_OK=Yes
+. git-sh-setup
+cd_to_toplevel
+require_work_tree
+
 if ! test "$#" -eq 7
 then
 	echo "$LONG_USAGE"
@@ -132,7 +137,7 @@ case "${1:-.}${2:-.}${3:-.}" in
 
 	# Create the working tree file, using "our tree" version from the
 	# index, and then store the result of the merge.
-	git checkout-index -f --stage=2 -- "$4" && cat "$src1" >"$4"
+	git checkout-index -f --stage=2 -- "$4" && cat "$src1" >"$4" || exit 1
 	rm -f -- "$orig" "$src1" "$src2"
 
 	if [ "$6" != "$7" ]; then
diff --git a/t/t6060-merge-index.sh b/t/t6060-merge-index.sh
new file mode 100755
index 0000000000000000000000000000000000000000..debadbd299c02630097a3f476729719d4d71a10c
--- /dev/null
+++ b/t/t6060-merge-index.sh
@@ -0,0 +1,100 @@
+#!/bin/sh
+
+test_description='basic git merge-index / git-merge-one-file tests'
+. ./test-lib.sh
+
+test_expect_success 'setup diverging branches' '
+	for i in 1 2 3 4 5 6 7 8 9 10; do
+		echo $i
+	done >file &&
+	git add file &&
+	git commit -m base &&
+	git tag base &&
+	sed s/2/two/ <file >tmp &&
+	mv tmp file &&
+	git commit -a -m two &&
+	git tag two &&
+	git checkout -b other HEAD^ &&
+	sed s/10/ten/ <file >tmp &&
+	mv tmp file &&
+	git commit -a -m ten &&
+	git tag ten
+'
+
+cat >expect-merged <<'EOF'
+1
+two
+3
+4
+5
+6
+7
+8
+9
+ten
+EOF
+
+test_expect_success 'read-tree does not resolve content merge' '
+	git read-tree -i -m base ten two &&
+	echo file >expect &&
+	git diff-files --name-only --diff-filter=U >unmerged &&
+	test_cmp expect unmerged
+'
+
+test_expect_success 'git merge-index git-merge-one-file resolves' '
+	git merge-index git-merge-one-file -a &&
+	git diff-files --name-only --diff-filter=U >unmerged &&
+	>expect &&
+	test_cmp expect unmerged &&
+	test_cmp expect-merged file &&
+	git cat-file blob :file >file-index &&
+	test_cmp expect-merged file-index
+'
+
+test_expect_success 'setup bare merge' '
+	git clone --bare . bare.git &&
+	(cd bare.git &&
+	 GIT_INDEX_FILE=$PWD/merge.index &&
+	 export GIT_INDEX_FILE &&
+	 git read-tree -i -m base ten two
+	)
+'
+
+test_expect_success 'merge-one-file fails without a work tree' '
+	(cd bare.git &&
+	 GIT_INDEX_FILE=$PWD/merge.index &&
+	 export GIT_INDEX_FILE &&
+	 test_must_fail git merge-index git-merge-one-file -a
+	)
+'
+
+test_expect_success 'merge-one-file respects GIT_WORK_TREE' '
+	(cd bare.git &&
+	 mkdir work &&
+	 GIT_WORK_TREE=$PWD/work &&
+	 export GIT_WORK_TREE &&
+	 GIT_INDEX_FILE=$PWD/merge.index &&
+	 export GIT_INDEX_FILE &&
+	 git merge-index git-merge-one-file -a &&
+	 git cat-file blob :file >work/file-index
+	) &&
+	test_cmp expect-merged bare.git/work/file &&
+	test_cmp expect-merged bare.git/work/file-index
+'
+
+test_expect_success 'merge-one-file respects core.worktree' '
+	mkdir subdir &&
+	git clone . subdir/child &&
+	(cd subdir &&
+	 GIT_DIR=$PWD/child/.git &&
+	 export GIT_DIR &&
+	 git config core.worktree "$PWD/child" &&
+	 git read-tree -i -m base ten two &&
+	 git merge-index git-merge-one-file -a &&
+	 git cat-file blob :file >file-index
+	) &&
+	test_cmp expect-merged subdir/child/file &&
+	test_cmp expect-merged subdir/file-index
+'
+
+test_done