file-upload - a4j:commandButton 通过 t:inputFileUpload 给出 NullPointerException

Question

为什么对于下面的代码，当我使用它时t:commandButton它工作正常并且万一a4j:commandButton它给出NullPointerException了uploadedFile.getInputStream()？

<h:form enctype="multipart/form-data">
    <t:inputFileUpload value="#{fileUploadController.uploadedFile}"/>
    <t:commandButton action="#{fileUploadController.insertFile}"/>      
</h:form>