diff --git a/freeze.go b/freeze.go
index 83457bd..3f2d20b 100644
--- a/freeze.go
+++ b/freeze.go
@@ -332,7 +332,7 @@ func NewServerFromFrozen(filename string) (s *Server, err os.Error) {
 		u.LastActive = fu.LastActive
 
 		s.Users[u.Id] = u
-		if u.Id > s.nextUserId {
+		if u.Id >= s.nextUserId {
 			s.nextUserId = u.Id + 1
 		}
 		s.UserNameMap[u.Name] = u
diff --git a/message.go b/message.go
index 4e989f9..25e9a15 100644
--- a/message.go
+++ b/message.go
@@ -803,13 +803,14 @@ func (server *Server) handleUserStateMessage(client *Client, msg *Message) {
 
 	userRegistrationChanged := false
 	if userstate.UserId != nil {
-		uid := server.RegisterClient(client)
-		if uid > 0 {
+		uid, err := server.RegisterClient(client)
+		if err != nil {
+			client.Printf("Unable to register: %v", err)
+			userstate.UserId = nil
+		} else {
 			userstate.UserId = proto.Uint32(uid)
 			client.user = server.Users[uid]
 			userRegistrationChanged = true
-		} else {
-			userstate.UserId = nil
 		}
 		broadcast = true
 	}
diff --git a/server.go b/server.go
index 2d8d92d..06c9fbf 100644
--- a/server.go
+++ b/server.go
@@ -155,6 +155,7 @@ func NewServer(id int64, addr string, port int) (s *Server, err os.Error) {
 
 	s.Users[0], err = NewUser(0, "SuperUser")
 	s.UserNameMap["SuperUser"] = s.Users[0]
+	s.nextUserId = 1
 
 	s.Channels = make(map[int]*Channel)
 	s.aclcache = NewACLCache()
@@ -1053,22 +1054,22 @@ func (s *Server) FreezeServer() io.ReadCloser {
 }
 
 // Register a client on the server.
-func (s *Server) RegisterClient(client *Client) (uid uint32) {
+func (s *Server) RegisterClient(client *Client) (uid uint32, err os.Error) {
 	// Increment nextUserId only if registration succeeded.
 	defer func() {
-		if uid > 0 {
+		if err == nil {
 			s.nextUserId += 1
 		}
 	}()
 
 	user, err := NewUser(s.nextUserId, client.Username)
 	if err != nil {
-		return 0
+		return 0, err
 	}
 
 	// Grumble can only register users with certificates.
 	if len(client.CertHash) == 0 {
-		return 0
+		return 0, os.NewError("no cert hash")
 	}
 
 	user.Email = client.Email
@@ -1078,7 +1079,8 @@ func (s *Server) RegisterClient(client *Client) (uid uint32) {
 	s.Users[uid] = user
 	s.UserCertMap[client.CertHash] = user
 	s.UserNameMap[client.Username] = user
-	return uid
+
+	return uid, nil
 }
 
 // Remove a registered user.