Unpacker throws exception trying to read map length after IsMapHeader returns true #320

I've implemented a custom MessagePackSerializer, and when trying to deserialize my type from the byte stream, which is expected to be a Map, I get an exception.

At the point of the exception unpacker.IsMapHeader returns true, but trying to do unpacker.ReadMapLength(...) throws:
MsgPack.MessageTypeException: Cannot convert 'map' header from type 'FixedRaw'(0xA6) in position 1.