diff --git a/src/py65/monitor.py b/src/py65/monitor.py
index fa6bc92..80966aa 100644
--- a/src/py65/monitor.py
+++ b/src/py65/monitor.py
@@ -328,7 +328,11 @@ class Monitor(cmd.Cmd):
           start += bytes
 
     def do_disassemble(self, args):
-        start, end = self._address_parser.range(args)
+        split = shlex.split(args)
+        if len(split) != 1:
+            return self.help_disassemble()
+
+        start, end = self._address_parser.range(split[0])
         if start == end:
             end += 1
 
@@ -350,6 +354,7 @@ class Monitor(cmd.Cmd):
     def help_disassemble(self):
         self._output("disassemble <address_range>")
         self._output("Disassemble instructions in the address range.")
+        self._output('Range is specified like "<start>:<end+1>".')
 
     def help_step(self):
         self._output("step")
diff --git a/src/py65/tests/test_monitor.py b/src/py65/tests/test_monitor.py
index 496cc3c..bf8d508 100644
--- a/src/py65/tests/test_monitor.py
+++ b/src/py65/tests/test_monitor.py
@@ -246,6 +246,20 @@ class MonitorTests(unittest.TestCase):
         out = stdout.getvalue()
         self.assertTrue(out.startswith('disassemble'))
 
+    def test_help_disassemble(self):
+        stdout = StringIO()
+        mon = Monitor(stdout=stdout)
+        mon.help_disassemble()
+        out = stdout.getvalue()
+        self.assertTrue(out.startswith('disassemble <address_range>'))
+
+    def test_disassemble_shows_help_when_given_extra_args(self):
+        stdout = StringIO()
+        mon = Monitor(stdout=stdout)
+        mon.do_disassemble("c000 c001")
+        out = stdout.getvalue()
+        self.assertTrue(out.startswith('disassemble <address_range>'))
+
     # fill
 
     def test_shortcut_f_for_fill(self):