DB entity service return null when calling from another entity

I have a service on a database entity, executing a simple select max() operation, when running in the entity, it works and returns a result (number) as shown in the screenshot. but when I call this service from another db entity as below: 

    lastTickTimeStamp = Things["Egress_KCT_FTAE"].Get_Last_TicksTimeStamp_Test();

it always return null as lastTickTimeStamp;

However, the other serivce (query to return an infotable) on the Things["Egress_KCT_FTAE"] can work in the same location without problem.