diff --git a/gateway/gateway.go b/gateway/gateway.go
index 732c9bcb..6fe19963 100644
--- a/gateway/gateway.go
+++ b/gateway/gateway.go
@@ -1,6 +1,7 @@
 package gateway
 
 import (
+	"os"
 	"regexp"
 	"strings"
 	"time"
@@ -138,6 +139,10 @@ func (gw *Gateway) mapChannelConfig(cfg []config.Bridge, direction string) {
 		if strings.HasPrefix(br.Account, "irc.") {
 			br.Channel = strings.ToLower(br.Channel)
 		}
+		if strings.HasPrefix(br.Account, "mattermost.") && strings.HasPrefix(br.Channel, "#") {
+			flog.Errorf("Mattermost channels do not start with a #: remove the # in %s", br.Channel)
+			os.Exit(1)
+		}
 		ID := br.Channel + br.Account
 		if _, ok := gw.Channels[ID]; !ok {
 			channel := &config.ChannelInfo{Name: br.Channel, Direction: direction, ID: ID, Options: br.Options, Account: br.Account,