diff --git a/src/py65/monitor.py b/src/py65/monitor.py
index 8007301..5cf7c86 100644
--- a/src/py65/monitor.py
+++ b/src/py65/monitor.py
@@ -597,9 +597,14 @@ class Monitor(cmd.Cmd):
     def help_mem(self):
         self._output("mem <address_range>")
         self._output("Display the contents of memory.")
+        self._output('Range is specified like "<start:end>".')
 
     def do_mem(self, args):
-        start, end = self._address_parser.range(args)
+        split = shlex.split(args)
+        if len(split) != 1:
+            return self.help_mem()
+
+        start, end = self._address_parser.range(split[0])
 
         line = self.addrFmt % start + ":"
         for address in range(start, end+1):
diff --git a/src/py65/tests/test_monitor.py b/src/py65/tests/test_monitor.py
index a9487f4..142b2bd 100644
--- a/src/py65/tests/test_monitor.py
+++ b/src/py65/tests/test_monitor.py
@@ -441,7 +441,23 @@ class MonitorTests(unittest.TestCase):
         mon.do_help('m')
 
         out = stdout.getvalue()
-        self.assertTrue(out.startswith('mem'))
+        self.assertTrue(out.startswith('mem <address_range>'))
+
+    def test_do_mem_shows_help_when_given_no_args(self):
+        stdout = StringIO()
+        mon = Monitor(stdout=stdout)
+        mon.do_mem('')
+
+        out = stdout.getvalue()
+        self.assertTrue(out.startswith('mem <address_range>'))
+
+    def test_do_mem_shows_help_when_given_extra_args(self):
+        stdout = StringIO()
+        mon = Monitor(stdout=stdout)
+        mon.do_mem('c000 c001')
+
+        out = stdout.getvalue()
+        self.assertTrue(out.startswith('mem <address_range>'))
 
     # mpu