diff --git a/server/user.go b/server/user.go
index ebbb2d0..5dd7c21 100644
--- a/server/user.go
+++ b/server/user.go
@@ -56,8 +56,7 @@ func replyCreateUser(s *Session, msg *ClientComMessage, rec *auth.Rec) {
 	}
 	if err != nil || !secretMatches {
 		logs.Warn.Println("create user: invalid registration account fields, sid=", s.sid)
-		s.queueOut(decodeStoreError(types.ErrPolicy, msg.Id, msg.Timestamp,
-			map[string]any{"what": "account", "reason": "REGISTRATION_ACCOUNT_INVALID"}))
+		s.queueOut(decodeStoreError(types.ErrPolicy, msg.Id, msg.Timestamp, registrationAccountErrorParams(err)))
 		return
 	}
 	authSecret := msg.Acc.Secret
@@ -407,6 +406,7 @@ var (
 	errRegistrationTenantForbidden = errors.New("registration tenant forbidden")
 	errRegistrationTenantRequired  = errors.New("registration tenant required")
 	errRegistrationTenantMismatch  = errors.New("registration tenant mismatch")
+	errRegistrationUsernameInvalid = errors.New("registration username invalid")
 	errLoginTypeRequired           = errors.New("login_type required")
 	errLoginTypeInvalid            = errors.New("login_type invalid")
 )
@@ -579,13 +579,22 @@ func loginTenantErrorParams(err error) map[string]any {
 	return params
 }
 
+func registrationAccountErrorParams(err error) map[string]any {
+	params := map[string]any{"what": "account", "reason": "REGISTRATION_ACCOUNT_INVALID"}
+	if errors.Is(err, errRegistrationUsernameInvalid) {
+		params["what"] = "username"
+		params["reason"] = "USERNAME_INVALID_FORMAT"
+	}
+	return params
+}
+
 func validateRegistrationUsername(username string) error {
 	if username == "" || len(username) > 10 {
-		return types.ErrPolicy
+		return errRegistrationUsernameInvalid
 	}
 	for i := range username {
 		if (username[i] < 'a' || username[i] > 'z') && (username[i] < '0' || username[i] > '9') {
-			return types.ErrPolicy
+			return errRegistrationUsernameInvalid
 		}
 	}
 	return nil
diff --git a/server/user_registration_test.go b/server/user_registration_test.go
index c9a448a..6d0aab0 100644
--- a/server/user_registration_test.go
+++ b/server/user_registration_test.go
@@ -71,6 +71,18 @@ func TestResolveRegistrationAccount(t *testing.T) {
 	}
 }
 
+func TestRegistrationAccountErrorParams(t *testing.T) {
+	params := registrationAccountErrorParams(errRegistrationUsernameInvalid)
+	if params["what"] != "username" || params["reason"] != "USERNAME_INVALID_FORMAT" {
+		t.Fatalf("got params=%v", params)
+	}
+
+	params = registrationAccountErrorParams(types.ErrPolicy)
+	if params["what"] != "account" || params["reason"] != "REGISTRATION_ACCOUNT_INVALID" {
+		t.Fatalf("got fallback params=%v", params)
+	}
+}
+
 func TestRegistrationSecretMatchesSelectedAccount(t *testing.T) {
 	if !registrationSecretMatches([]byte("alice:password:with-colon"), "alice") {
 		t.Fatal("matching account was rejected")