diff --git a/harold/plugins/slack.py b/harold/plugins/slack.py
index 4cb9556..695b090 100644
--- a/harold/plugins/slack.py
+++ b/harold/plugins/slack.py
@@ -275,6 +275,7 @@ def replace_channel_mention(m):
             channel = channels_by_name[channel_name]
         except KeyError:
             print("Attempted to send message to unknown channel: %s" % channel_name)
+            return
 
         try:
             yield self._api_client.make_request(