diff --git a/src/testdir/test_json.vim b/src/testdir/test_json.vim
index 0248aa9..3ee7837 100644
--- a/src/testdir/test_json.vim
+++ b/src/testdir/test_json.vim
@@ -107,6 +107,9 @@
   call assert_equal('"café"', json_encode("caf\xe9"))
   let &encoding = save_encoding
 
+  " Invalid utf-8 sequences are replaced with U+FFFD (replacement character)
+  call assert_equal('"foo' . "\ufffd" . '"', json_encode("foo\xAB"))
+
   call assert_fails('echo json_encode(function("tr"))', 'E1161: Cannot json encode a func')
   call assert_fails('echo json_encode([function("tr")])', 'E1161: Cannot json encode a func')