Unable to extract value from ml entity in microsoft bot framework composer

Using bot framework composer connected to LUIS. If I use a prebuilt entity eg @prebuilt personName, and user input is My name is {personName}, the name can be extracted by using the expression =@personName in the value field.

But if I use a @ml entity ie @ml theName and in the value field use the expression =@theName, the name isn't extracted from the My name is {theName}.

I'm clearly missing something - but why can you extract from prebuilt entities and not ml entities using =@entityname in the value field? How do you do it with ml entities?

Thx