NodaTime UnparsableValueException due to usage of "Z" in pattern

I am exchanging JSON messages between Java and C# (and vice-versa).

In Java I use a java.time.Instant (JSR-310) to represent a point in time on the global timeline. In order to create a human readable date/time string in JSON, I convert my Instant as follows:

private static final DateTimeFormatter FORMATTER = ofPattern("yyyy-MM-dd'T'HH:mm:ssZ").withZone(ZoneId.systemDefault());

which generates the following output:

2017-04-28T19:54:44-0500

Now, on the message consumer side of things (C#) I wrote a custom Newtonsoft.Json.JsonConverter, which extends the abstract JsonCreationConvert class that contains the following overridden ReadJson() method:

public override object ReadJson(JsonReader reader, Type objectType, object existingValue,
        JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
        {
            return null;
        }

        if (reader.TokenType == JsonToken.StartArray)
        {
            return JToken.Load(reader).ToObject<string[]>();
        }

        reader.DateParseHandling = DateParseHandling.None; // read NodaTime string Instant as is
        serializer.Converters.Add(NodaConverters.InstantConverter);

        // Load JObject from stream
        var jObject = JObject.Load(reader);

        // Create target object based on JObject
        T target = Create(objectType, jObject);

        // Populate the object properties
        var writer = new StringWriter();
        serializer.Serialize(writer, jObject);
        using (var newReader = new JsonTextReader(new StringReader(writer.ToString())))
        {
            newReader.Culture = reader.Culture;
            newReader.DateParseHandling = reader.DateParseHandling;
            newReader.DateTimeZoneHandling = reader.DateTimeZoneHandling;
            newReader.FloatParseHandling = reader.FloatParseHandling;
            serializer.Populate(newReader, target);
        }

        return target;
    }

Create() is an abstract method.

When I now convert this JSON string into a NodaTime.Instant (v2.0.0) by calling:

InstantPattern.General.Parse(creationTime).Value;

I get this exception:

NodaTime.Text.UnparsableValueException: The value string does not match a quoted string in the pattern. Value being parsed: '2017-04-28T19:54:44^-0500'. (^ indicates error position.)

If I pass a text literal "Z" (so no outputted offset "-0500" and Z is interpreted as 0 offset) the NodaTime.Serialization.JsonNet.NodaConverters.InstantConverter correctly reads without throwing an exception.

Looking into the GeneralPatternImpl I see:

internal static readonly InstantPattern GeneralPatternImpl = InstantPattern.CreateWithInvariantCulture("uuuu-MM-ddTHH:mm:ss'Z'");

Why does an InstantConverter require the offset to be a text literal? Is this happening because an Instant is agnostic to an offset? If this is the case, then why doesn't the InstantConverter just ignore the offset instead of throwing an exception? Do I need to write a custom converter to get around this problem?