Master/Detail Pull

I'm trying to add a Subquery to a mapping where the mapped object is the Master in a Master/Detail object.

I have a custom events object Event__c that is mapped and pulling fine - I need to pull data from the custom object Event_Facility_Members__c where Event__C is the master.

I have the soql like this:

$query->fields[] =
          "(SELECT Facility__c from Event_Facility_Members__c WHERE Event_Facility_Members__c.Event__C = '$query->fields[0]')";

I've also tried it with a Event__c record Id like this that I know should return results:

$query->fields[] =
          "(SELECT Facility__c from Event_Facility_Members__c WHERE Event_Facility_Members__c.Event__C = 'a0KEa00000B07CrMAJ')";

In every instance I get an error in line 217 of the queuehandler Call to a member function size() on null.

I assume it's a SOQL issue - but I can't figure it out as the SOQL works in the developer console in SF.