Does not implement methodSignatureForSelector - trouble ahead

In iOS if you get the following error:

does not implement methodSignatureForSelector: — trouble ahead

It's most likely that one of your classes does not inherit from NSObject.  An easy fix is to changes this:

@interface MyClass

to this:

@interface MyClass : NSObject

iOS 5 seems to be able to handle when a class does not inherit from NSObject.  iOS 4 and lower will throw this error.