diff --git a/src/lib/Server/Output/ValueObjectVisitor/RestContentType.php b/src/lib/Server/Output/ValueObjectVisitor/RestContentType.php
index ddca2c20..e0d34dc8 100644
--- a/src/lib/Server/Output/ValueObjectVisitor/RestContentType.php
+++ b/src/lib/Server/Output/ValueObjectVisitor/RestContentType.php
@@ -127,7 +127,7 @@ public function visit(Visitor $visitor, Generator $generator, $data)
 
         $generator->startValueElement(
             'isContainer',
-            $this->serializeBool($generator, $contentType->isContainer)
+            $this->serializeBool($generator, $contentType->isContainer())
         );
         $generator->endValueElement('isContainer');