XML "simple" count of Node items returning error

Hi,

I have a Repeated Section in Nintex Forms for Office 365 and for now I just want to extract the number of repeated items in that section just as follows;

REPEATED SECTION TEXT ONCE FORM IS STORED:

<?xml version="1.0" encoding="utf-8"?>

<RepeaterData>

  <Version />

  <Items>

  <Item>

  <ItemTitle_x0020_ type="System.String">this is title 1</ItemTitle_x0020_>

  </Item>

  <Item>

  <ItemTitle_x0020_ type="System.String">this is title 2</ItemTitle_x0020_>

  </Item>

</Items>

</RepeaterData>

Workflow Action Query XML only does this:    count(//RepeaterData/Items/Item)   (screenshot)

And when ran the Workflow log shows me an error "The request cannot be processed due to internal error" but nothing else.

any idea ?

screenshot of the error:

thanks,

Francois