Error: Invalid or no assignment to system datatype found

Hi All,

I am trying to get some data from the back-end with SAP Gateway. I have created a data model, generated and implemented the get_entityset for my entity. When i call the service, the output data is filled correctly after the get_entityset is executed.

The problem appears when the Gateway tries to convert the data into JSON format. An exception is thrown and an error appears in the Gateway log too, which states that there is an invalid or no assignment to system datatype. I checked the data model in the Gateway, regenerated the runtime artifacts, but I still get the same error.

This is my data model:

Any ideas what the problem might be?

Thanks,

Szilamér