Can I return a struct from a WebMethod

Question

I have a struct called Base64String which basically does implicit conversion between a string and a byte[]

It's working quite well in the library, but here's the oddity...

When I have a webservice, that returns the value as Base64String

    [WebMethod]
    public Base64String GetBase64String(string input)
    {
        return input;
    }

This method was merely a test though, as Base64String is the type of a property on the real object being returned.

And then Add a Reference to the webservice in another project, it has a return type of DataSet??? Why is it doing that? Of all the types it could pick, why DataSet, it's not even remotely close to what I want.

And for reference the Base64String struct:

using System.Linq;
using System.Collections.Generic;
using System.Xml;
using System.Xml.Schema;
using System.Xml.Serialization;
using System;
using System.Text;

[XmlRoot("string")]
public struct Base64String : IEqualityComparer<Base64String>, IXmlSerializable
{
    private byte[] bytes;

    private Base64String(byte[] value)
    {
        bytes = value;
    }

    private Base64String(string value)
    {
        bytes = Convert.FromBase64String(value);
    }

    public static Base64String FromText(string text)
    {
        return new Base64String(Encoding.UTF8.GetBytes(text));
    }

    public static implicit operator string(Base64String value)
    {
        return value.bytes == null ? string.Empty : Convert.ToBase64String(value.bytes);
    }

    public static implicit operator byte[](Base64String value)
    {
        return value.bytes;
    }

    public static implicit operator Base64String(string value)
    {
        return new Base64String(value);
    }

    public static implicit operator Base64String(byte[] value)
    {
        return new Base64String(value);
    }

    public static bool operator ==(Base64String left, Base64String right)
    {
        // Both null, thus equal
        if ((object)left == null && (object)right == null)
            return true;

        // Only one is null, thus not equal
        if ((object)left == null || (object)right == null)
            return false;

        // Neither null, compare
        return left.bytes.SequenceEqual(right.bytes);
    }

    public static bool operator !=(Base64String left, Base64String right)
    {
        return !(left == right);
    }

    public override bool Equals(object obj)
    {
        return this == (Base64String)obj;
    }

    public override int GetHashCode()
    {
        return base.GetHashCode();
    }

    public bool Equals(Base64String x, Base64String y)
    {
        return (x == y);
    }

    public int GetHashCode(Base64String obj)
    {
        return base.GetHashCode();
    }

    public override string ToString()
    {
        return this;
    }

    public XmlSchema GetSchema()
    {
        return null;
    }

    public void ReadXml(XmlReader reader)
    {
        this = reader.ReadString();
    }

    public void WriteXml(XmlWriter writer)
    {
        writer.WriteString(this);
    }
}