Reading document with embedded dictionary who's values have a dictionary fails to load the child dictionary

Am using C# and the Mongo C# driver, I’m successfully writing a document to mongo that has dictionary keyed by string, and who’s values have a dictionary which has string keys.

When I try read the document back with an IMongoCollection.Find(Expression) I get the higher level dictionary , but the dictionary in its value is always null.

How do I ensure I’m reading all the data in the document?