Why won't my object serialize? - asp.net-web-api2

Question

I have a class. It implements IXmlSerializable. I can't get Web API 2 to serialize it.
Here is my class:
public class MyType: IXmlSerializable
{
public XmlSchema GetSchema()
{
return null;
}
public void WriteXml(XmlWriter writer)
{
// some code
}
public void ReadXml(XmlReader reader)
{
throw new NotImplementedException();
}
}
Here is the response:
<Error>
<Message>An error has occurred.</Message>
<ExceptionMessage>
The 'ObjectContent`1' type failed to serialize the response body for content type 'text/xml; charset=utf-8'.
</ExceptionMessage>
<ExceptionType>System.InvalidOperationException</ExceptionType>
<StackTrace />
<InnerException>
<Message>An error has occurred.</Message>
<ExceptionMessage>Type 'MyType' with data contract name 'XmlConverter:http://schemas.datacontract.org/2004/07/MyNamespace' is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.</ExceptionMessage>
<ExceptionType>System.Runtime.Serialization.SerializationException</ExceptionType>
<StackTrace>
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeAndVerifyType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, Boolean verifyKnownType, RuntimeTypeHandle declaredTypeHandle, Type declaredType)
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiTypeAtTopLevel(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle originalDeclaredTypeHandle, Type graphType)
at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
at System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
at System.Runtime.Serialization.DataContractSerializer.WriteObject(XmlWriter writer, Object graph)
at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, HttpContent content)
at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStreamAsync(Type type, Object value, Stream writeStream, HttpContent content, TransportContext transportContext, CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.WebHost.HttpControllerHandler.<WriteBufferedResponseContentAsync>d__1b.MoveNext()
</StackTrace>
</InnerException>
</Error>
I've tried [DataContract]. That changes the 'Type 'MyNamespace.MyType' cannot be IXmlSerializable and have DataContractAttribute attribute.' I've tried using [XmlSchemaProvider("Method", IsAny = true)], with Method returning null, which gives me the original error again. DataContractSerializer is supposed to be able to serialize IXmlSerializables. What gives?
P.S.: WriteXml is not being called at all.