Been bit by this a couple of times now. It’s really a problem with a bad route. In my case I had some ajax using the FOSJsRoutingBundle to generate a route along the lines of:

Routing.generate('some_titles', {id: $(this).val()});

where val() was actually returning an empty string. The resulting URL (“some//titles”) was invalid, looks to JMSSerializer like a resource thanks to the double slash, and badda bing badda boom: completely irrelevant error.

I think this may also be the source of “Cannot redeclare class Doctrine\ORM\Mapping\Annotation” errors to boot. Double whammy.