diff --git a/irc/reply.go b/irc/reply.go
index 8f1af885..7df09b25 100644
--- a/irc/reply.go
+++ b/irc/reply.go
@@ -15,8 +15,14 @@ func joinedLen(names []string) int {
 }
 
 type BaseReply struct {
-	source  Identifier
+	id      string
 	message string
+	source  Identifier
+}
+
+func (reply *BaseReply) SetSource(source Identifier) {
+	reply.id = source.Id()
+	reply.source = source
 }
 
 func (reply *BaseReply) Source() Identifier {
@@ -33,22 +39,22 @@ func NewStringReply(source Identifier, code string,
 	reply := &StringReply{
 		code: code,
 	}
+	reply.SetSource(source)
 	reply.message = fmt.Sprintf(format, args...)
-	reply.source = source
 	return reply
 }
 
 func (reply *StringReply) Format(client *Client) []string {
 	message := fmt.Sprintf("%s %s", reply.code, reply.message)
 	if Identifier(client.server) != reply.source {
-		message = fmt.Sprintf(":%s %s", reply.source.Id(), message)
+		message = fmt.Sprintf(":%s %s", reply.id, message)
 	}
 	return []string{message}
 }
 
 func (reply *StringReply) String() string {
 	return fmt.Sprintf("Reply(source=%s, code=%s, message=%s)",
-		reply.source, reply.code, reply.message)
+		reply.id, reply.code, reply.message)
 }
 
 type NumericReply struct {
@@ -61,8 +67,8 @@ func NewNumericReply(source Identifier, code int, format string,
 	reply := &NumericReply{
 		code: code,
 	}
+	reply.SetSource(source)
 	reply.message = fmt.Sprintf(format, args...)
-	reply.source = source
 	return reply
 }
 
@@ -70,14 +76,14 @@ func (reply *NumericReply) Format(client *Client) []string {
 	message := fmt.Sprintf("%03d %s %s",
 		reply.code, client.Nick(), reply.message)
 	if Identifier(client.server) != reply.source {
-		message = fmt.Sprintf(":%s %s", reply.source.Id(), message)
+		message = fmt.Sprintf(":%s %s", reply.id, message)
 	}
 	return []string{message}
 }
 
 func (reply *NumericReply) String() string {
 	return fmt.Sprintf("Reply(source=%s, code=%d, message=%s)",
-		reply.source, reply.code, reply.message)
+		reply.id, reply.code, reply.message)
 }
 
 // names reply
@@ -88,12 +94,11 @@ type NamesReply struct {
 }
 
 func NewNamesReply(channel *Channel) Reply {
-	return &NamesReply{
-		BaseReply: &BaseReply{
-			source: channel,
-		},
+	reply := &NamesReply{
 		channel: channel,
 	}
+	reply.SetSource(channel)
+	return reply
 }
 
 func (reply *NamesReply) Format(client *Client) []string {
@@ -401,3 +406,7 @@ func ErrChanOPrivIsNeeded(channel *Channel) Reply {
 func ErrNoMOTD(server *Server) Reply {
 	return NewNumericReply(server, ERR_NOMOTD, ":MOTD File is missing")
 }
+
+func ErrNoNicknameGiven(server *Server) Reply {
+	return NewNumericReply(server, ERR_NONICKNAMEGIVEN, ":No nickname given")
+}
diff --git a/irc/server.go b/irc/server.go
index aeca8847..7eb018d8 100644
--- a/irc/server.go
+++ b/irc/server.go
@@ -233,11 +233,17 @@ func (m *PassCommand) HandleServer(s *Server) {
 func (m *NickCommand) HandleServer(s *Server) {
 	c := m.Client()
 
+	if m.nickname == "" {
+		c.Reply(ErrNoNicknameGiven(s))
+		return
+	}
+
 	if s.clients[m.nickname] != nil {
 		c.Reply(ErrNickNameInUse(s, m.nickname))
 		return
 	}
 
+	// Make reply before changing nick.
 	reply := RplNick(c, m.nickname)
 
 	s.clients.Remove(c)
diff --git a/irc/types.go b/irc/types.go
index d62f4826..2cf20b86 100644
--- a/irc/types.go
+++ b/irc/types.go
@@ -1,6 +1,7 @@
 package irc
 
 import (
+	"errors"
 	"fmt"
 )
 
@@ -51,9 +52,17 @@ func (channels ChannelNameMap) Remove(channel *Channel) error {
 
 type ClientNameMap map[string]*Client
 
+var (
+	ErrNickMissing   = errors.New("nick missing")
+	ErrNicknameInUse = errors.New("nickname in use")
+)
+
 func (clients ClientNameMap) Add(client *Client) error {
+	if !client.HasNick() {
+		return ErrNickMissing
+	}
 	if clients[client.nick] != nil {
-		return fmt.Errorf("%s: already set", client.nick)
+		return ErrNicknameInUse
 	}
 	clients[client.nick] = client
 	return nil