Swift: ObjectiveC RunTime unrecognized copyWithZone error for value types

Question

I am looking to migrate some code from Swift -> Objective C starting with unit tests.

I am having issues with mocking functions that passes value types:

let initWithTitleBlock : @objc_block (String, String) -> AlertViewMock! = { (title : String, message : String) -> (AlertViewMock!) in
    alert = AlertViewMock(title: title, message: message)
    return alert
}
let initWithTitleImp = imp_implementationWithBlock(unsafeBitCast(initWithTitleBlock, AnyObject.self))
let initWithTitleMethod = class_getInstanceMethod(AlertView.self, Selector("initWithTitle:message:"))
method_setImplementation(initWithTitleMethod, initWithTitleImp)

But I end up getting the following error at runtime:

-[AlertView copyWithZone:]: unrecognized selector sent to instance 0x7fef006e4cb0

However, if I change the type getting passed into the block as AnyObject or some other reference type like NSURL, I am able to successfully run the new block (except I cannot convert the object back into a string):

let initWithTitleBlock : @objc_block (AnyObject, AnyObject) -> AlertViewMock! = { (title : AnyObject, message : AnyObject) -> (AlertViewMock!) in ...

I did not mark AlertViewMock as copy. May anyone help explain what's going on? If I pass in NSInteger, I get the same exception:

let initWithTitleBlock : @objc_block (NSInteger, NSInteger) -> AlertViewMock! = { (title : NSInteger, message : NSInteger) -> (AlertViewMock!) in ...