Groovy Script Works when Testing Custom Script, but not in Orchestrator Client

Hi All

I have a Groovy Custom Script that takes the output from a data request and runs through it to apply some "advanced" filtering that can't be achieved in the data request itself. To test it within the script editor I have grabbed a copy of the output from the data request and it does exactly what I expect.

However, when I try to test the complete Orchestration in Orchestrator Client it gives me the error:

"POC_CUST_ExpenseCostCentresFilter:Exception: NullPointerException, Message: Cannot get property 'rowset' on null object"

The output from the data request starts with the following layout: