diff --git a/merge-recursive.c b/merge-recursive.c
index 9fb0b9f8fd..be01f528cb 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -1952,6 +1952,13 @@ int merge_trees(struct merge_options *o,
 	}
 
 	if (oid_eq(&common->object.oid, &merge->object.oid)) {
+		struct strbuf sb = STRBUF_INIT;
+
+		if (index_has_changes(&sb)) {
+			err(o, _("Dirty index: cannot merge (dirty: %s)"),
+			    sb.buf);
+			return 0;
+		}
 		output(o, 0, _("Already up to date!"));
 		*result = head;
 		return 1;
diff --git a/t/t6044-merge-unrelated-index-changes.sh b/t/t6044-merge-unrelated-index-changes.sh
index 5e472be92b..23b86fb977 100755
--- a/t/t6044-merge-unrelated-index-changes.sh
+++ b/t/t6044-merge-unrelated-index-changes.sh
@@ -112,7 +112,7 @@ test_expect_success 'recursive' '
 	test_must_fail git merge -s recursive C^0
 '
 
-test_expect_failure 'recursive, when merge branch matches merge base' '
+test_expect_success 'recursive, when merge branch matches merge base' '
 	git reset --hard &&
 	git checkout B^0 &&