diff --git a/src/Auth/Db.php b/src/Auth/Db.php
index b3b288e..a6d694e 100644
--- a/src/Auth/Db.php
+++ b/src/Auth/Db.php
@@ -138,18 +138,29 @@ class Db extends Backend {
    */
   public static function add_user($info) {
     $values = [
-      App::get('auth.db.username_field') => $info['username'],
-      App::get('auth.db.password_field') => password_hash(
-        $info['password'],
-        constant('PASSWORD_'.strtoupper(App::get('auth.db.password_hash_algo')))
+      self :: $username_field => $info['username'] ?? null,
+      self :: $password_field => (
+        ($info['password'] ?? null)?
+        password_hash(
+          $info['password'],
+          constant('PASSWORD_'.strtoupper(App::get('auth.db.password_hash_algo')))
+        ):
+        null
       ),
     ];
     foreach($info as $field => $value) {
       if (!$value) {
-        Log :: error("add_user: field %s is missing", $field);
+        Log :: error("add_user: field %s is missing (or null)", $field);
         return false;
       }
     }
+
+    // Check username uniqueness
+    if (self :: get_user($info['username'])) {
+      Log :: error("add_user: a user with username %s already exist");
+      return false;
+    }
+
     foreach(App :: get('auth.db.exposed_fields') as $field)
       if (isset($info[$field]) && $info[$field])
         $values[$field] = $info[$field];