突然得到编译器崩溃“arrayForceCast”在Swift XCode beta 6

Undefined symbols for architecture i386:
  "__TFSs15_arrayForceCastU___FGSaQ__GSaQ0__",referenced from:
      __TFC9MyProj6NavBar5stackfS0_FCSo20CDVInvokedUrlCommandT_ in NavBar.o

我设法追溯到这行代码，这在beta 5上工作得很好。

// A dictionary of String keys and arrays of NavBarItem instances as values.
var stacks: [String:[NavBarItem]] = [:]

func stack(cmd: CDVInvokedUrlCommand) {
    let stackName: String = cmd.arguments[0] as String

    if let stack = stacks[stackName] {

        // When I comment this line out I get a clean compile.
        navBar.setItems(stack,animated: false)

    }
}

我唯一的猜想是，它不喜欢从一个swift数组转换到一个NSArray不知何故。有任何想法吗？