IncludeDataField doesn't work with nullable fields if the mapping has other data

[DrSmugleaf]

Leaving the fields for SpriteSpecifier.Rsi out of a mapping that has other data leads to this error for example:

Robust.Shared.Serialization.InvalidMappingException: Expected sprite-node
   at Robust.Shared.Serialization.TypeSerializers.Implementations.SpriteSpecifierSerializer.Robust.Shared.Serialization.TypeSerializers.Interfaces.ITypeReader<Robust.Shared.Utility.SpriteSpecifier.Rsi,Robust.Shared.Serialization.Markdown.Mapping.MappingDataNode
>.Read(ISerializationManager serializationManager, MappingDataNode node, IDependencyCollection dependencies, SerializationHookContext hookCtx, ISerializationContext context, InstantiationDelegate`1 instanceProvider) in SpriteSpecifierSerializer.cs:line 68

[ElectroJr]

Is this true for all data defs, or specific to sprite specifiers? Because they use a SpriteSpecifierSerializer, not the normal data definition serializers, which is pretty uncommon for data definitions, so its quite possible that this issue is related to that.