The report could not be generated.

No applicable method 'StartsWith' exists in type 'DynamicClass13'

I'm using a modified zero value order lines report to get invoice item descriptions and their respective invoice numbers into columns. When I try to apply a filter on the item description so that only the invoice lines that have start with a certain word are shown I get the above message. Can anyone help with this? I'm using the original joins, have just changed the layout to simplify the data for easier working on Excel.