NSScanner字符串始终为nil（EXC_BAD_INSTRUCTION（代码= EXC_i386_INVOP，子代码= 0x0））

Question

使用NSScanner时，代码如下：

var s: String?
var b: NSString?
let scn = NSScanner(string: "andgostring")
scn.scanUpToString("go", intoString: &b)
b = NSString(string:"http")
s = b // <--- Crashes here 

...与

崩溃

  

EXC_BAD_INSTRUCTION（代码= EXC_i386_INVOP，子码=为0x0