Investigating an assertionFailure

This afternoon I was testing my Swift/Spritekit game on my iPad when it crashed. I've recovered the crash log fine, but at a bit of a loss as to the problem, as I'm not sure what assertion actually failed. Here is the top of the log...

Exception Codes: 0x0000000000000001, 0x00000001b1d7dd7c
Termination Reason: SIGNAL 5 Trace/BPT trap: 5
Terminating Process: exc handler [524]

Triggered by Thread:  0

Thread 0 name:   Dispatch queue:
Thread 0 Crashed:
0   libswiftCore.dylib            	       0x1b1d7dd7c _assertionFailure(_:_:file:line:flags:) + 312
1   libswiftCore.dylib            	       0x1b1d7dd7c _assertionFailure(_:_:file:line:flags:) + 311
2   MyGame                        	       0x102d28f50 CampaignState.healNonChosenActors() + 948
3   MyGame                        	       0x102c08f50 specialized MissionReportScene.init(actors:mission:) + 6968

If I understand correctly, EXC_BREAKPOINT (SIGTRAP) and Trace/BPT trap: 5 points at a Swift assertion failing, but the code in question isn't doing any force-unwrapping, potentially overflowing arithmetic or anything else fishy, literally just a filter of a non-optional array followed by a foreach.

Is there a way to figure out exactly what the runtime thought went wrong?

Problem solved - some internet sleuthing suggested that this was probably down to some guard clauses failing and hitting a fatalError(...). Following back from there I found an edge case and fixed it.