New Post: Rollup Function Issue

I am using the rollup function with the follow string:

<fetch distinct="false" mapping="logical" output-format="xml-platform" version="1.0"><entity name="opportunity"><attribute name="name"/><attribute name="opportunityid"/><order descending="false" attribute="name"/></entity></fetch>

I keep getting the following exception any ideas as to what is not formatted properly?

Unexpected exception from plug-in (Execute): msdyncrmWorkflowTools.RollupFunctions: System.FormatException: Input string was not in a correct format.