diff --git a/diff-lib.c b/diff-lib.c
index ae91027a02..a23119b852 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -467,7 +467,7 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	if (cached && idx && ce_stage(idx)) {
 		struct diff_filepair *pair;
 		pair = diff_unmerge(&revs->diffopt, idx->name);
-		if (tree)
+		if (pair && tree)
 			fill_filespec(pair->one, &tree->oid, 1,
 				      tree->ce_mode);
 		return;
diff --git a/t/t4045-diff-relative.sh b/t/t4045-diff-relative.sh
index 2c8493fe66..167be0bdcc 100755
--- a/t/t4045-diff-relative.sh
+++ b/t/t4045-diff-relative.sh
@@ -245,4 +245,13 @@ test_expect_failure 'diff --relative with change in subdir' '
 	test_cmp expected out
 '
 
+test_expect_success 'diff --relative --cached with change in subdir' '
+	git switch br3 &&
+	test_when_finished "git merge --abort" &&
+	test_must_fail git merge sub1 &&
+	echo file0 >expected &&
+	git -C subdir diff --relative --name-only --cached >out &&
+	test_cmp expected out
+'
+
 test_done