Primary key property has duplicate values after migration - Realm migration

Question

I have an existing Realm project with two models

Model1 -> id, updateDate, details
Model2 -> id, updateDate, title, model1

(yes I used a class object instead of the id - aargh. Both id are primary keys)

With an updated version of my app, I am adding a new property to Model1 (title) and changing Model2.model1 from type Model1 to type string (=Model1.id)

I wrote a migration block for this as per the samples provided

let migrationBlock: (RLMMigration, UInt64) -> Void = { (migration, oldSchemeVersion) in
    if oldSchemeVersion < 1 {
        migration.enumerateObjects(Model1.className()) { oldObject, newObject in
            //Nothing needed, the title can be a blank
        }
        migration.enumerateObjects(Model2.className()) { oldObject, newObject in
            if let oldModel1 = oldObject!["model1"] as? RLMDynamicObject {
                newObject!["model1"] = oldModel1["id"]
            }
        }
    }
}

let config = RLMRealmConfiguration.defaultConfiguration()
config.schemaVersion = newSchemaVersion
config.migrationBlock = migrationBlock
RLMRealmConfiguration.setDefaultConfiguration(config)

But at the end of the migration, when I try to access the default realm (Realm.defaultRealm), it fails with this error

Terminating app due to uncaught exception 'RLMException', reason: 'Primary key property 'id' has duplicate values after migration.'

I cannot figure out why this is going wrong and what I am supposed to do to make this work. Any help would be appreciated.

NOTE - my code uses the Realm objective-c code but in a Swift app