把NSURL从NSUserDefauls放到NSSet

Question

为什么以下代码会引发错误：

NSUserDefaults *sharedDefaults = [[NSUserDefaults alloc] initWithSuiteName:@"com.apple.example"];
NSURL *m = [sharedDefaults URLForKey:@"test"]; // just another fileURLWithPath

NSURL *c = [NSURL fileURLWithPath:@"/Users/dummy/Documents"];
NSURL *d = [NSURL fileURLWithPath:@"/Users/dummy/Pictures"];
NSSet *q = [NSSet setWithObjects:c, d, m, nil];

特别是，错误是：

Service exited due to signal: Segmentation fault: 11