diff --git a/src/py65/monitor.py b/src/py65/monitor.py
index 40c66bb..89451c7 100644
--- a/src/py65/monitor.py
+++ b/src/py65/monitor.py
@@ -627,7 +627,7 @@ class Monitor(cmd.Cmd):
         split = shlex.split(args)
         if len(split) != 2:
             self._output("Syntax error: %s" % args)
-            return
+            return self.help_add_label()
 
         address = self._address_parser.number(split[0])
         label   = split[1]
diff --git a/src/py65/tests/test_monitor.py b/src/py65/tests/test_monitor.py
index d75cb69..a93f9d6 100644
--- a/src/py65/tests/test_monitor.py
+++ b/src/py65/tests/test_monitor.py
@@ -55,7 +55,8 @@ class MonitorTests(unittest.TestCase):
         mon = Monitor(stdout=stdout)
         mon.do_add_label('should be label space value')
         out = stdout.getvalue()
-        self.assertEqual("Syntax error: should be label space value\n", out)
+        err = "Syntax error: should be label space value\n"
+        self.assertTrue(out.startswith(err))
 
     def test_do_add_label_adds_label(self):
         stdout = StringIO()