diff --git a/lib/redmine_mattermost/listener.rb b/lib/redmine_mattermost/listener.rb
index e6659e7..8a379c8 100644
--- a/lib/redmine_mattermost/listener.rb
+++ b/lib/redmine_mattermost/listener.rb
@@ -268,6 +268,10 @@ private
 	end
 
 	def extract_usernames text = ''
+		if text.nil?
+			text = ''
+		end
+
 		# mattermost usernames may only contain lowercase letters, numbers,
 		# dashes and underscores and must start with a letter or number.
 		text.scan(/@[a-z0-9][a-z0-9_\-]*/).uniq