diff --git a/filters/gentoo-ldap-authentication.lua b/filters/gentoo-ldap-authentication.lua
index c1e382f..3b6564b 100644
--- a/filters/gentoo-ldap-authentication.lua
+++ b/filters/gentoo-ldap-authentication.lua
@@ -106,7 +106,7 @@ local lualdap = require("lualdap")
 
 function gentoo_ldap_user_groups(username, password)
 	-- Ensure the user is alphanumeric
-	if username:match("%W") then
+	if username == nil or username:match("%W") then
 		return nil
 	end