PrimeFaces <p:blockUI> not working with <p:poll>
===

I am using PrimeFaces JSF library and I have some trouble using `<p:blockUI>` component. 

More preciselly, I can sucessfully block whole form ("formTranscodingPage"). Using the following code I block it every 5 seconds with `<p:poll>`:

    <h:form id="formTranscodingPage">
      .
      .
      .
      <p:scrollPanel id="scrollFilesId"...>       
         <p:tree ...>
         </p:tree>
         <p:poll id="pollId" interval="5"... />
      </p:scrollPanel>
      .
      .
      .
      <p:blockUI block=":formTranscodingPage" trigger="pollId">
        <p:graphicImage value="/images/loader.gif"/> 
      </p:blockUI>
      .
      .
      .
    </h:form>


But when I want to block only `<p:scrollPanel>` ("scrollFilesId") - **it does not work**:

      <p:blockUI block=":formTranscodingPage:scrollFilesId" trigger="pollId">
        <p:graphicImage value="/images/loader.gif"/> 
      </p:blockUI>

Am I doing something wrong?