StringMap - Peristence.mod - error

[GWRon]

I just replaced TPeristence's Field objectMap:TMap to use TStringMap.

When now loading in a savegame of my game I get one of the Reference not mapped yet-errors thrown. Did I miss to change something?
Reverting back to TMap = new TMap and loading is successful.

[GWRon]

Using TPersistence with a TStringMap to serialize stuff, works - unserializing throws the error. Unserializing with TMap works too.