Query works fine in NW Designer but not in workflow

I have a simple query that finds the last item in a list (same site) and it works fine in the Nintex Workflow Designer,  but errors when it is run in a workflow...the error simply states "error" no clue as to what is actually failing.

 <Query>
  <Lists>
    <List ID="{1DA8FDBD-DA2F-43A3-99B7-E6303E46889E}" />
  </Lists>
  <ViewFields>
    <FieldRef Name="PreviousBalance" />
  </ViewFields>
  <OrderBy>
    <FieldRef Name="ID" Ascending="false" />
  </OrderBy>
</Query>

Which returns:

PreviousBalance:
 172,281

Works perfectly when I run it inside of the query designer but the query fails with "error" when the workflow runs. I am running the query against a local list which I have full control permissions and still get the error,  tried assigning the output to a single line of text instead of a number variable, to no avail ?

Any ideas would be really helpful !