diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 9cae4a3..b98c840 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -1104,7 +1104,7 @@
       assert_equal([], getbufline(b, 2, 1))
 
       if has('job')
-        setbufline(b, 2, [function('eval'), {key: 123}, test_null_job()])
+        setbufline(b, 2, [function('eval'), {key: 123}, string(test_null_job())])
         assert_equal(["function('eval')",
                         "{'key': 123}",
                         "no process"],
@@ -1250,6 +1250,16 @@
   actual->assert_equal(expected)
 enddef
 
+def Test_substitute()
+  var res = substitute('A1234', '\d', 'X', '')
+  assert_equal('AX234', res)
+
+  if has('job')
+    assert_fails('"text"->substitute(".*", () => job_start(":"), "")', 'E908: using an invalid value as a String: job')
+    assert_fails('"text"->substitute(".*", () => job_start(":")->job_getchannel(), "")', 'E908: using an invalid value as a String: channel')
+  endif
+enddef
+
 def Test_synID()
   new
   setline(1, "text")