Puzzling Core Data crasher

gistfile1.m

- (id)insertObjectOfEntityName:(NSString *)entityName configure:(void(^)(id))block
{
  NSAssert([NSThread isMainThread], @"Wrong thread bro");
  NSManagedObject *object = [NSEntityDescription insertNewObjectForEntityForName:entityName inManagedObjectContext:self.mainQueueContext];
  if (block) block(object);
  NSError *error = nil;
  if (![self.mainQueueContext save:&error]) {
    NSLog(@"Error saving MOC: %@", error);
  }
  return object;
}

- (void)someMethod
{
  [self insertObjectOfEntityName:@"Something" configure:^(Something *obj) {
    // managedObjectContext is nil at this point, which causes things to go very very wrong with the
    // rest of the code in this block...
    NSLog(@"%@", obj.managedObjectContext); 
  }];
}

That's what I'm trying right now (don't have enough info to report back on whether that worked) but it was a shot in the dark and I'm still confused as to what's actually going on. mainQueueContext is confirmed to not be nil, and a strong reference is held for the lifetime of the app.

EDIT: I should also mention that this actually works properly 99% of the time, but it does crash once every now and then.