How to inherit from NSDictionary?

I have an object called Settings that inherits from NSMutableDictionary. When I try to initialize this object using

Settings *settings = [[Settings alloc] initWithContentsOfFile: @"someFile"]

it returns an object of type NSCFDictionary. As a result, it doesn't recognize my additional methods. For example, when I call the selector "save", it objects:

[NSCFDictionary save]: unrecognized selector sent to instance 0x524bc0

Of course, it's OK when I initialize using the garden variety

Settings *settings = [[Settings alloc] init]

I tried to cast it again to Settings but that didn't work. This seems really simple - what am I missing?

Thanks