diff --git a/diff.c b/diff.c
index 1054a4b732..c78ea27033 100644
--- a/diff.c
+++ b/diff.c
@@ -4281,7 +4281,7 @@ static void add_external_diff_name(struct repository *r,
 				   const char *name,
 				   struct diff_filespec *df)
 {
-	struct diff_tempfile *temp = prepare_temp_file(r, name, df);
+	struct diff_tempfile *temp = prepare_temp_file(r, df->path, df);
 	strvec_push(argv, temp->name);
 	strvec_push(argv, temp->hex);
 	strvec_push(argv, temp->mode);
diff --git a/t/t4045-diff-relative.sh b/t/t4045-diff-relative.sh
index fab351b48a..5e1bbb57e7 100755
--- a/t/t4045-diff-relative.sh
+++ b/t/t4045-diff-relative.sh
@@ -162,6 +162,35 @@ check_diff_relative_option subdir file2 true --no-relative --relative
 check_diff_relative_option . file2 false --no-relative --relative=subdir
 check_diff_relative_option . file2 true --no-relative --relative=subdir
 
+test_expect_success 'external diff with --relative' '
+	test_when_finished "git reset --hard" &&
+	echo changed >file1 &&
+	echo changed >subdir/file2 &&
+
+	write_script mydiff <<-\EOF &&
+	# hacky pretend diff; the goal here is just to make sure we got
+	# passed sensible input that we _could_ diff, without relying on
+	# the specific output of a system diff tool.
+	echo "diff a/$1 b/$1" &&
+	echo "--- a/$1" &&
+	echo "+++ b/$1" &&
+	echo "@@ -1 +0,0 @@" &&
+	sed "s/^/-/" "$2" &&
+	sed "s/^/+/" "$5"
+	EOF
+
+	cat >expect <<-\EOF &&
+	diff a/file2 b/file2
+	--- a/file2
+	+++ b/file2
+	@@ -1 +0,0 @@
+	-other content
+	+changed
+	EOF
+	GIT_EXTERNAL_DIFF=./mydiff git diff --relative=subdir >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'setup diff --relative unmerged' '
 	test_commit zero file0 &&
 	test_commit base subdir/file0 &&