diff --git a/src/JsonMapperHelper.php b/src/JsonMapperHelper.php
index 355867f..4f736bd 100644
--- a/src/JsonMapperHelper.php
+++ b/src/JsonMapperHelper.php
@@ -21,7 +21,7 @@ public static function setUndefinedProperty($object, $propName, $jsonValue)
                 $object->{$propName} = $jsonValue;
                 $object->customFields[$propName] = $jsonValue;
             }
-        } else {
+        } elseif (isset($object->{$propName})) {
             $object->{$propName} = $jsonValue;
         }
     }