diff --git a/http2irc.py b/http2irc.py
index f36d179..9de99df 100644
--- a/http2irc.py
+++ b/http2irc.py
@@ -353,6 +353,8 @@ class IRCClientProtocol(asyncio.Protocol):
 
 	def send(self, data):
 		self.logger.debug(f'Send: {data!r}')
+		if len(data) > 510:
+			raise RuntimeError(f'IRC message too long ({len(data)} > 510): {data!r}')
 		self.transport.write(data + b'\r\n')
 
 	async def _get_message(self):