diff --git a/irc/client.go b/irc/client.go
index 9fc041ae..462deeb8 100644
--- a/irc/client.go
+++ b/irc/client.go
@@ -623,12 +623,15 @@ func (client *Client) HasUsername() bool {
 	return client.username != "" && client.username != "*"
 }
 
+// SetNames sets the client's ident and realname.
 func (client *Client) SetNames(username, realname string) error {
-	usernameCasefolded, err := CasefoldName(username)
-	if err != nil {
+	// do this before casefolding to ensure these are actually ascii
+	if !isIdent(username) {
 		return errInvalidUsername
 	}
 
+	usernameCasefolded := strings.ToLower(username) // only ascii is supported in idents anyway
+
 	client.stateMutex.Lock()
 	defer client.stateMutex.Unlock()
 
diff --git a/irc/handlers.go b/irc/handlers.go
index 7fadeffa..e89fee37 100644
--- a/irc/handlers.go
+++ b/irc/handlers.go
@@ -2195,8 +2195,7 @@ func userHandler(server *Server, client *Client, msg ircmsg.IrcMessage, rb *Resp
 
 	err := client.SetNames(msg.Params[0], msg.Params[3])
 	if err == errInvalidUsername {
-		rb.Add(nil, "", "ERROR", client.t("Malformed username"))
-		return true
+		rb.Add(nil, server.name, ERR_INVALIDUSERNAME, client.t("Malformed username"))
 	}
 
 	return false
diff --git a/irc/numerics.go b/irc/numerics.go
index ec8e83cc..05c4ff21 100644
--- a/irc/numerics.go
+++ b/irc/numerics.go
@@ -143,6 +143,7 @@ const (
 	ERR_YOUREBANNEDCREEP            = "465"
 	ERR_YOUWILLBEBANNED             = "466"
 	ERR_KEYSET                      = "467"
+	ERR_INVALIDUSERNAME             = "468"
 	ERR_CHANNELISFULL               = "471"
 	ERR_UNKNOWNMODE                 = "472"
 	ERR_INVITEONLYCHAN              = "473"
diff --git a/irc/strings.go b/irc/strings.go
index 0e5e50ca..5ef67e78 100644
--- a/irc/strings.go
+++ b/irc/strings.go
@@ -128,6 +128,32 @@ func isBoring(name string) bool {
 	return true
 }
 
+// returns true if the given name is a valid ident, using a mix of Insp and
+// Chary's ident restrictions.
+func isIdent(name string) bool {
+	if len(name) < 1 {
+		return false
+	}
+
+	for i := 0; i < len(name); i++ {
+		chr := name[i]
+		if (chr >= 'a' && chr <= 'z') || (chr >= 'A' && chr <= 'Z') || (chr >= '0' && chr <= '9') {
+			continue // alphanumerics
+		}
+		if i == 0 {
+			return false // first char must be alnum
+		}
+		switch chr {
+		case '[', '\\', ']', '^', '_', '{', '|', '}', '-', '.', '`':
+			continue // allowed chars
+		default:
+			return false // disallowed chars
+		}
+	}
+
+	return true
+}
+
 // Skeleton produces a canonicalized identifier that tries to catch
 // homoglyphic / confusable identifiers. It's a tweaked version of the TR39
 // skeleton algorithm. We apply the skeleton algorithm first and only then casefold,
diff --git a/irc/strings_test.go b/irc/strings_test.go
index 6b60a0f0..757722d4 100644
--- a/irc/strings_test.go
+++ b/irc/strings_test.go
@@ -140,6 +140,22 @@ func TestIsBoring(t *testing.T) {
 	assertBoring("Νικηφόρος", false)
 }
 
+func TestIsIdent(t *testing.T) {
+	assertIdent := func(str string, expected bool) {
+		if isIdent(str) != expected {
+			t.Errorf("expected [%s] to have identness [%t], but got [%t]", str, expected, !expected)
+		}
+	}
+
+	assertIdent("warning", true)
+	assertIdent("sid3225", true)
+	assertIdent("dan.oak25", true)
+	assertIdent("dan.oak[25]", true)
+	assertIdent("phi@#$%ip", false)
+	assertIdent("Νικηφόρος", false)
+	assertIdent("-dan56", false)
+}
+
 func TestSkeleton(t *testing.T) {
 	skeleton := func(str string) string {
 		skel, err := Skeleton(str)