将Swift Dictionary转换为NSDictionary

var params = ["first_name": firstNameTextField.text,"last_name": lastNameTextField.text,"company": companyTextField.text,"email_address": emailTextField.text,"phone_number": phoneTextField.text]

接下来,我有ObjC框架,我连接到我的Swift应用程序.
里面有一个属性对象

@property (nonatomic,retain) NSMutableDictionary *fields;

我试图以这种方式分配它

object.fields = params

并得到一个错误：

Cannot convert the expression's type '()' to type 'NSMutableDictionary!'

我也试过这样做

var params = ["first_name": firstNameTextField.text,"phone_number": pnoneTextField.text].mutableCopy() as NSMutableDictionary

object.fields = params

编译得很好,但我遇到了运行时错误：

*** Terminating app due to uncaught exception 'NSInvalidArgumentException',reason: '*** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: 
attempt to insert nil object from objects[0]'

我打印了所有字段,但它们不是零：

println(firstNameTextField.text)
println(lastNameTextField.text)
println(companyTextField.text)
println(emailTextField.text)
println(pnoneTextField.text)

...

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有任何想法吗？