diff --git a/irc/channel.go b/irc/channel.go
index f9b16138..6c59a777 100644
--- a/irc/channel.go
+++ b/irc/channel.go
@@ -224,7 +224,7 @@ func (channel *Channel) Mode(client *Client, changes ChannelModeChanges) {
 			switch change.op {
 			case Add:
 				if change.arg == "" {
-					// TODO err reply
+					client.ErrNeedMoreParams("MODE")
 					continue
 				}
 
@@ -243,18 +243,18 @@ func (channel *Channel) Mode(client *Client, changes ChannelModeChanges) {
 			}
 
 			if change.arg == "" {
-				// TODO err reply
+				client.ErrNeedMoreParams("MODE")
 				continue
 			}
 
 			target := channel.server.clients.Get(change.arg)
 			if target == nil {
-				// TODO err reply
+				client.ErrNoSuchNick(change.arg)
 				continue
 			}
 
-			if channel.members[target] == nil {
-				// TODO err reply
+			if !channel.members.Has(target) {
+				client.ErrUserNotInChannel(channel, target)
 				continue
 			}
 
@@ -267,6 +267,9 @@ func (channel *Channel) Mode(client *Client, changes ChannelModeChanges) {
 				channel.members[target][change.mode] = false
 				applied = append(applied, change)
 			}
+
+		default:
+			client.ErrUnknownMode(change.mode, channel)
 		}
 	}
 
diff --git a/irc/reply.go b/irc/reply.go
index d729aa61..90678ba7 100644
--- a/irc/reply.go
+++ b/irc/reply.go
@@ -10,18 +10,28 @@ func NewStringReply(source Identifier, code StringCode,
 	format string, args ...interface{}) string {
 	var header string
 	if source == nil {
-		header = fmt.Sprintf("%s ", code)
+		header = code.String() + " "
 	} else {
 		header = fmt.Sprintf(":%s %s ", source, code)
 	}
-	message := fmt.Sprintf(format, args...)
+	var message string
+	if len(args) > 0 {
+		message = fmt.Sprintf(format, args...)
+	} else {
+		message = format
+	}
 	return header + message
 }
 
 func NewNumericReply(target *Client, code NumericCode,
 	format string, args ...interface{}) string {
 	header := fmt.Sprintf(":%s %s %s ", target.server.Id(), code, target.Nick())
-	message := fmt.Sprintf(format, args...)
+	var message string
+	if len(args) > 0 {
+		message = fmt.Sprintf(format, args...)
+	} else {
+		message = format
+	}
 	return header + message
 }
 
@@ -406,3 +416,8 @@ func (target *Client) ErrErroneusNickname(nick string) {
 	target.NumericReply(ERR_ERRONEUSNICKNAME,
 		"%s :Erroneous nickname", nick)
 }
+
+func (target *Client) ErrUnknownMode(mode ChannelMode, channel *Channel) {
+	target.NumericReply(ERR_UNKNOWNMODE,
+		"%s :is unknown mode char to me for %s", mode, channel)
+}