diff --git a/irc/accounts.go b/irc/accounts.go
index e573ed04..e1e5c54c 100644
--- a/irc/accounts.go
+++ b/irc/accounts.go
@@ -381,7 +381,7 @@ func (am *AccountManager) Register(client *Client, account string, callbackNames
 		return errAccountCreation
 	}
 
-	if restrictedCasefoldedNicks[casefoldedAccount] || restrictedSkeletons[skeleton] {
+	if restrictedCasefoldedNicks.Has(casefoldedAccount) || restrictedSkeletons.Has(skeleton) {
 		return errAccountAlreadyRegistered
 	}
 
diff --git a/irc/client_lookup_set.go b/irc/client_lookup_set.go
index 1f4fbe82..b0b82d86 100644
--- a/irc/client_lookup_set.go
+++ b/irc/client_lookup_set.go
@@ -173,7 +173,7 @@ func (clients *ClientManager) SetNick(client *Client, session *Session, newNick
 			return "", errNicknameInvalid, false
 		}
 
-		if restrictedCasefoldedNicks[newCfNick] || restrictedSkeletons[newSkeleton] {
+		if restrictedCasefoldedNicks.Has(newCfNick) || restrictedSkeletons.Has(newSkeleton) {
 			return "", errNicknameInvalid, false
 		}
 
diff --git a/irc/nickname.go b/irc/nickname.go
index c44d7635..150ccb5b 100644
--- a/irc/nickname.go
+++ b/irc/nickname.go
@@ -23,8 +23,8 @@ var (
 		"MemoServ", "BotServ", "OperServ",
 	}
 
-	restrictedCasefoldedNicks = make(map[string]bool)
-	restrictedSkeletons       = make(map[string]bool)
+	restrictedCasefoldedNicks = make(utils.StringSet)
+	restrictedSkeletons       = make(utils.StringSet)
 )
 
 func performNickChange(server *Server, client *Client, target *Client, session *Session, nickname string, rb *ResponseBuffer) error {
diff --git a/irc/services.go b/irc/services.go
index ee5712e3..9fe7dd19 100644
--- a/irc/services.go
+++ b/irc/services.go
@@ -350,11 +350,11 @@ func initializeServices() {
 		if err != nil {
 			panic(err)
 		}
-		restrictedCasefoldedNicks[cfName] = true
+		restrictedCasefoldedNicks.Add(cfName)
 		skeleton, err := Skeleton(restrictedNickname)
 		if err != nil {
 			panic(err)
 		}
-		restrictedSkeletons[skeleton] = true
+		restrictedSkeletons.Add(skeleton)
 	}
 }