Check if value is set before serializing

src/Alias/Model.php

@@ -79,6 +79,9 @@ abstract class Model implements ModelInterface
                 continue;
             }
             $p = strtolower(str_replace('get', '', $method->getName()));
+            if (!isset($this->{$p})) {
+                continue;
+            }
             $output[$p] = $this->{$method->getName()}();
         }
         return $output;