diff --git a/src/ircmsgs.py b/src/ircmsgs.py
index 4c746448b..2d7e6232c 100644
--- a/src/ircmsgs.py
+++ b/src/ircmsgs.py
@@ -532,6 +532,7 @@ def kick(channel, nick, s='', prefix='', msg=None):
         assert isNick(nick), repr(nick)
     if msg and not prefix:
         prefix = msg.prefix
+    assert isinstance(s, str)
     if s:
         return IrcMsg(prefix=prefix, command='KICK',
                       args=(channel, nick, s), msg=msg)
@@ -547,6 +548,7 @@ def kicks(channel, nicks, s='', prefix='', msg=None):
         assert all(isNick, nicks), nicks
     if msg and not prefix:
         prefix = msg.prefix
+    assert isinstance(s, str)
     if s:
         return IrcMsg(prefix=prefix, command='KICK',
                       args=(channel, ','.join(nicks), s), msg=msg)
@@ -559,6 +561,7 @@ def privmsg(recipient, s, prefix='', msg=None):
     if conf.supybot.protocols.irc.strictRfc():
         assert (isChannel(recipient) or isNick(recipient)), repr(recipient)
         assert s, 's must not be empty.'
+    assert isinstance(s, str)
     if msg and not prefix:
         prefix = msg.prefix
     return IrcMsg(prefix=prefix, command='PRIVMSG',
@@ -588,6 +591,7 @@ def notice(recipient, s, prefix='', msg=None):
     if conf.supybot.protocols.irc.strictRfc():
         assert (isChannel(recipient) or isNick(recipient)), repr(recipient)
         assert s, 'msg must not be empty.'
+    assert isinstance(s, str)
     if msg and not prefix:
         prefix = msg.prefix
     return IrcMsg(prefix=prefix, command='NOTICE', args=(recipient, s), msg=msg)
@@ -635,6 +639,7 @@ def part(channel, s='', prefix='', msg=None):
         assert isChannel(channel), repr(channel)
     if msg and not prefix:
         prefix = msg.prefix
+    assert isinstance(s, str)
     if s:
         return IrcMsg(prefix=prefix, command='PART',
                       args=(channel, s), msg=msg)
@@ -648,6 +653,7 @@ def parts(channels, s='', prefix='', msg=None):
         assert all(isChannel, channels), channels
     if msg and not prefix:
         prefix = msg.prefix
+    assert isinstance(s, str)
     if s:
         return IrcMsg(prefix=prefix, command='PART',
                       args=(','.join(channels), s), msg=msg)
@@ -674,6 +680,7 @@ def topic(channel, topic=None, prefix='', msg=None):
         return IrcMsg(prefix=prefix, command='TOPIC',
                       args=(channel,), msg=msg)
     else:
+        assert isinstance(topic, str)
         return IrcMsg(prefix=prefix, command='TOPIC',
                       args=(channel, topic), msg=msg)