JSF - Ajax render only if element exists

I have the following usecase: I have a button on the screen which rerenders an other element.

<h:commandButton value="button">
  <f:ajax  execute="@this" render="toBeRendered" />
</h:commandButton>
......
<h:outputPanel id="toBeRendered"/>

My problem is that this other element (toBeRendered) does not always exists so JSF fails when it builds the view and can't find the element thats in the render attribute of the f:ajax tag. I know that this validation is no longer there in newer version of mojarra but updating is not an option. Does anybody has a workaround that so that it would only render when the other element exists?