When attempting to serialize a Document with a float field, BsonWriter.TranslateToBsonType returns BsonDataType.Number for System.Single types.

In BsonWriter.WriteValue, an attempt is made to cast the object to double which is invalid since the object is of type System.Single:

case BsonDataType.Number:
writer.Write((double)obj);
return;

Here's a good article on the why http://blogs.msdn.com/ericlippert/archive/2009/03/19/representation-and-identity.aspx

One possible fix would be:

if (obj is System.Single) {
writer.Write((double)(float)obj);
} else {
writer.Write((double)obj);
}