diff --git a/diff.c b/diff.c
index 1054a4b732..3117fce39b 100644
--- a/diff.c
+++ b/diff.c
@@ -4213,7 +4213,6 @@ static void prep_temp_blob(struct index_state *istate,
 }
 
 static struct diff_tempfile *prepare_temp_file(struct repository *r,
-					       const char *name,
 					       struct diff_filespec *one)
 {
 	struct diff_tempfile *temp = claim_diff_tempfile();
@@ -4231,18 +4230,18 @@ static struct diff_tempfile *prepare_temp_file(struct repository *r,
 
 	if (!S_ISGITLINK(one->mode) &&
 	    (!one->oid_valid ||
-	     reuse_worktree_file(r->index, name, &one->oid, 1))) {
+	     reuse_worktree_file(r->index, one->path, &one->oid, 1))) {
 		struct stat st;
-		if (lstat(name, &st) < 0) {
+		if (lstat(one->path, &st) < 0) {
 			if (errno == ENOENT)
 				goto not_a_valid_file;
-			die_errno("stat(%s)", name);
+			die_errno("stat(%s)", one->path);
 		}
 		if (S_ISLNK(st.st_mode)) {
 			struct strbuf sb = STRBUF_INIT;
-			if (strbuf_readlink(&sb, name, st.st_size) < 0)
-				die_errno("readlink(%s)", name);
-			prep_temp_blob(r->index, name, temp, sb.buf, sb.len,
+			if (strbuf_readlink(&sb, one->path, st.st_size) < 0)
+				die_errno("readlink(%s)", one->path);
+			prep_temp_blob(r->index, one->path, temp, sb.buf, sb.len,
 				       (one->oid_valid ?
 					&one->oid : null_oid()),
 				       (one->oid_valid ?
@@ -4251,7 +4250,7 @@ static struct diff_tempfile *prepare_temp_file(struct repository *r,
 		}
 		else {
 			/* we can borrow from the file in the work tree */
-			temp->name = name;
+			temp->name = one->path;
 			if (!one->oid_valid)
 				oid_to_hex_r(temp->hex, null_oid());
 			else
@@ -4269,7 +4268,7 @@ static struct diff_tempfile *prepare_temp_file(struct repository *r,
 	else {
 		if (diff_populate_filespec(r, one, NULL))
 			die("cannot read data blob for %s", one->path);
-		prep_temp_blob(r->index, name, temp,
+		prep_temp_blob(r->index, one->path, temp,
 			       one->data, one->size,
 			       &one->oid, one->mode);
 	}
@@ -4278,10 +4277,9 @@ static struct diff_tempfile *prepare_temp_file(struct repository *r,
 
 static void add_external_diff_name(struct repository *r,
 				   struct strvec *argv,
-				   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);
 	strvec_push(argv, temp->name);
 	strvec_push(argv, temp->hex);
 	strvec_push(argv, temp->mode);
@@ -4308,11 +4306,9 @@ static void run_external_diff(const char *pgm,
 	strvec_push(&cmd.args, name);
 
 	if (one && two) {
-		add_external_diff_name(o->repo, &cmd.args, name, one);
-		if (!other)
-			add_external_diff_name(o->repo, &cmd.args, name, two);
-		else {
-			add_external_diff_name(o->repo, &cmd.args, other, two);
+		add_external_diff_name(o->repo, &cmd.args, one);
+		add_external_diff_name(o->repo, &cmd.args, two);
+		if (other) {
 			strvec_push(&cmd.args, other);
 			strvec_push(&cmd.args, xfrm_msg);
 		}
@@ -7037,7 +7033,7 @@ static char *run_textconv(struct repository *r,
 	struct strbuf buf = STRBUF_INIT;
 	int err = 0;
 
-	temp = prepare_temp_file(r, spec->path, spec);
+	temp = prepare_temp_file(r, spec);
 	strvec_push(&child.args, pgm);
 	strvec_push(&child.args, temp->name);
 
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 &&