diff --git a/irctest/cases.py b/irctest/cases.py
index 30b1aed..4d14b3a 100644
--- a/irctest/cases.py
+++ b/irctest/cases.py
@@ -160,6 +160,7 @@ class _IrcTestCase(Generic[TController]):
     def messageDiffers(
         self,
         msg: Message,
+        command: Union[str, None, patma.Operator] = None,
         params: Optional[List[Union[str, None, patma.Operator]]] = None,
         target: Optional[str] = None,
         tags: Optional[
@@ -186,6 +187,14 @@ class _IrcTestCase(Generic[TController]):
                     msg=msg,
                 )
 
+        if command is not None and not patma.match_string(msg.command, command):
+            fail_msg = (
+                fail_msg or "expected command to match {expects}, got {got}: {msg}"
+            )
+            return fail_msg.format(
+                *extra_format, got=msg.command, expects=command, msg=msg
+            )
+
         if prefix is not None and not patma.match_string(msg.prefix, prefix):
             fail_msg = (
                 fail_msg or "expected prefix to match {expects}, got {got}: {msg}"
@@ -214,7 +223,7 @@ class _IrcTestCase(Generic[TController]):
                     or "expected nick to be {expects}, got {got} instead: {msg}"
                 )
                 return fail_msg.format(
-                    *extra_format, got=got_nick, expects=nick, param=key, msg=msg
+                    *extra_format, got=got_nick, expects=nick, msg=msg
                 )
 
         return None
diff --git a/irctest/self_tests/cases.py b/irctest/self_tests/cases.py
index 428a73c..3613298 100644
--- a/irctest/self_tests/cases.py
+++ b/irctest/self_tests/cases.py
@@ -173,7 +173,7 @@ MESSAGE_SPECS: List[Tuple[Dict, List[str], List[str], List[str]]] = [
         ],
         # and they each error with:
         [
-            "expected command to be PRIVMSG, got PRIVMG",
+            "expected command to match PRIVMSG, got PRIVMG",
             "expected tags to match {'tag1': 'bar', RemainingKeys(ANYSTR): ANYOPTSTR}, got {'tag1': 'value1'}",
             "expected params to match ['#chan', 'hello'], got ['#chan', 'hello2']",
             "expected params to match ['#chan', 'hello'], got ['#chan2', 'hello']",
@@ -206,7 +206,7 @@ MESSAGE_SPECS: List[Tuple[Dict, List[str], List[str], List[str]]] = [
         ],
         # and they each error with:
         [
-            "expected command to be PRIVMSG, got PRIVMG",
+            "expected command to match PRIVMSG, got PRIVMG",
             "expected tags to match {StrRe(r'tag[12]'): 'bar', RemainingKeys(ANYSTR): ANYOPTSTR}, got {'tag1': 'value1'}",
             "expected params to match ['#chan', 'hello'], got ['#chan', 'hello2']",
             "expected params to match ['#chan', 'hello'], got ['#chan2', 'hello']",
@@ -235,7 +235,7 @@ MESSAGE_SPECS: List[Tuple[Dict, List[str], List[str], List[str]]] = [
         ],
         # and they each error with:
         [
-            "expected command to be PRIVMSG, got PRIVMG",
+            "expected command to match PRIVMSG, got PRIVMG",
             "expected tags to match {'tag1': 'bar', RemainingKeys(NotStrRe(r'tag2')): ANYOPTSTR}, got {'tag1': 'value1'}",
             "expected tags to match {'tag1': 'bar', RemainingKeys(NotStrRe(r'tag2')): ANYOPTSTR}, got {'tag1': 'bar', 'tag2': ''}",
             "expected tags to match {'tag1': 'bar', RemainingKeys(NotStrRe(r'tag2')): ANYOPTSTR}, got {'tag1': 'bar', 'tag2': 'baz'}",
@@ -345,7 +345,7 @@ MESSAGE_SPECS: List[Tuple[Dict, List[str], List[str], List[str]]] = [
         ],
         # and they each error with:
         [
-            "expected command to be PING, got PONG"
+            "expected command to match PING, got PONG"
         ]
     ),
 ]
diff --git a/irctest/server_tests/monitor.py b/irctest/server_tests/monitor.py
index f5bd808..aa417ec 100644
--- a/irctest/server_tests/monitor.py
+++ b/irctest/server_tests/monitor.py
@@ -8,6 +8,7 @@ import pytest
 from irctest import cases, runner
 from irctest.client_mock import NoMessageException
 from irctest.numerics import (
+    ERR_ERRONEUSNICKNAME,
     RPL_ENDOFMONLIST,
     RPL_MONLIST,
     RPL_MONOFFLINE,
@@ -190,14 +191,15 @@ class MonitorTestCase(_BaseMonitorTestCase):
         self.check_server_support()
         self.sendLine(1, "MONITOR + *!username@localhost")
         self.sendLine(1, "MONITOR + *!username@127.0.0.1")
+        expected_command = StrRe(f"({RPL_MONOFFLINE}|{ERR_ERRONEUSNICKNAME})")
         try:
             m = self.getMessage(1)
-            self.assertMessageMatch(m, command="731")
+            self.assertMessageMatch(m, command=expected_command)
         except NoMessageException:
             pass
         else:
             m = self.getMessage(1)
-            self.assertMessageMatch(m, command="731")
+            self.assertMessageMatch(m, command=expected_command)
         self.connectClient("bar")
         try:
             m = self.getMessage(1)