ReadAllQuery -- Missing Descriptor for Additional Data Store

I am following the guide from GitHub - cuba-labs/large-dataset: How to handle large data sets to improve the performance of my database-heavy application.
My application uses an additional datastore, which has client data that needs to be processed, so it is read only. I try to use the second option, which relies on EclipseLink’s ReadAllQuery class. When I use this process on the entities in my main datastore, I have no issues. However, when using my additional datastore (SQL Server), I get Missing Descriptor exception. Any ideas?