Retrieve Case object via jscript on related entity - undefined error

I'm trying to retrieve the Case on a related entity via jscript so that I can use the Case number to populate another field value. The case is a lookup on the custom entity. I'm using the following to retrieve the object but it throws an undefined error:

var case = Xrm.Page.getAttribute('abc_case').getValue();

I've used this same code elsewhere to do the same thing and it works fine. In fact I use it to pull an object from a custom entity in the same block of code. Is there something different about the case? If I take out that one line the code works fine.