diff --git a/diff-lib.c b/diff-lib.c
index 46cae637ec..ac4e310438 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;
@@ -530,6 +530,16 @@ static int oneway_diff(const struct cache_entry * const *src,
 	if (tree == o->df_conflict_entry)
 		tree = NULL;
 
+	/*
+	 * We should only see a NULL idx when the entry was present in the tree
+	 * but deleted in the idx. In which case it should be impossible
+	 * that a NULL tree was passed in (there would have been no entry at
+	 * all) or that we got a df conflict above (you need a directory and a
+	 * file to get such a conflict, which implies both sides are present).
+	 */
+	if (!idx && !tree)
+		BUG("oneway_diff with neither idx nor tree");
+
 	if (ce_path_match(revs->diffopt.repo->index,
 			  idx ? idx : tree,
 			  &revs->prune_data, NULL)) {
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