Incompatible block pointer... error when block

Question

I implemented MKInputBox, and I'm trying to use a block statement. Here is the code: (It's in their sample project:)

alertView.customise = ^(UITextField *textField) {

};

I get an error saying the following:

Incompatible block pointer types assigning to 'UITextField *(^)(UITextField *__strong)' from 'void (^)(UITextField *__strong)'

What am I doing wrong, and how can I fix that?