Wrong deserialization for dictionaries with Nullable<>-values

[evifox]

I wrote simple unit test which check values after deserialization and I got exception on checking value with key [2]:

var incoming = new Dictionary<int, int?>
    {
        [0] = null,
        [1] = 123,
        [2] = null,
    };
var serializer = new GroBuf.Serializer(
    new GroBuf.DataMembersExtracters.AllPropertiesExtractor(),
    new GroBuf.DefaultGroBufCustomSerializerCollection(),
    GroBuf.GroBufOptions.MergeOnRead | GroBuf.GroBufOptions.WriteEmptyObjects);
var raw = serializer.Serialize(incoming);
var outgoing = serializer.Deserialize<Dictionary<int, int?>>(raw);
outgoing[0].Should().Be(null);
outgoing[1].Should().Be(123);
outgoing[2].Should().Be(null); // Expected <null>, but found 123.

When I use nullable-values by default such as "string" then deserialization work correct.