symfony-jsonapi: Related Doctrine Entities fail with empty document?

When I try to serialize a doctrine that has related entities, I get a blank 500 error and have to dig through my logs to figure out what happened.

It looks like it’s boiling down to a nesting max depth error:

[Thu Nov 19 11:58:08.370490 2015] [:error] [pid 18288] [client 10.0.0.70:50204] PHP Fatal error:  Maximum function nesting level of '256' reached, aborting! in Unknown on line 0

Any ideas on serializing related Doctrine entities?