diff --git a/http2irc.py b/http2irc.py
index 85a7d1c..4498a4c 100644
--- a/http2irc.py
+++ b/http2irc.py
@@ -516,7 +516,7 @@ class IRCClientProtocol(asyncio.Protocol):
 	def _self_usermask_length(self):
 		if not self.server.nickname or not self.server.username or not self.server.hostname:
 			return 100
-		return len(self.server.nickname) + len(self.server.username) + len(self.server.hostname)
+		return len(self.server.nickname) + 1 + len(self.server.username) + 1 + len(self.server.hostname) # nickname!username@hostname
 
 	async def send_messages(self):
 		while self.connected:
@@ -527,7 +527,7 @@ class IRCClientProtocol(asyncio.Protocol):
 				break
 			channelB = channel.encode('utf-8')
 			messageB = message.encode('utf-8')
-			usermaskPrefixLength = 1 + self._self_usermask_length() + 1
+			usermaskPrefixLength = 1 + self._self_usermask_length() + 1 # :usermask<SP>
 			if usermaskPrefixLength + len(b'PRIVMSG ' + channelB + b' :' + messageB) > 510:
 				# Message too long, need to split or truncate. First try to split on spaces, then on codepoints. Ideally, would use graphemes between, but that's too complicated.
 				self.logger.debug(f'Message too long, overlongmode = {overlongmode}')