[__NSCFNumber length]: unrecognized selector sent to instance

If you get any of these runtime errors/warnings:
[__NSCFNumber length]: unrecognized selector sent to instance
restkit.object_mapping:RKMappingOperation.m: Coercing NSNull value to nil in shouldSetValue:atKeyPath: -- should be fixed

class User: NSObject {

    var userId: String?

    var userImageUrl: String?

}

Check whether server is sending JSON response not as String like
{
  "id":12345
  "avatar_url":""
}

Solution:

Such error causing properties should be defined as NSObject

class User: NSObject {

    var userId: NSObject?

    var userImageUrl: String?

}