What would cause the DataContractSerializer to throw the following exception?

System.Runtime.Serialization.SerializationException was unhandled by user code
Message="Type 'System.RuntimeType' with data contract name 'RuntimeType:http://schemas.datacontract.org/2004/07/System' is not expected. 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."

WHen it does succeed to serialize my object, ir produces the following XML;



I have removed all DataMember attributes on properties of the targeted class, yet the serializer still throws and exception, serializes backing fields for certain properties and ignores others. Any ideas on what is going on here? This is the XML created for the base class.

<d5p1:Filter xmlns:d6p1="http://schemas.datacontract.org/2004/07/OCC600.Infrastructure.Dictionary.BusinessEntities" i:type="d6p1:SitMessageFilter">
<d6p1:_x003C_Alias_x003E_k__BackingField i:nil="true" />
<d6p1:_x003C_EntityType_x003E_k__BackingField>None</d6p1:_x003C_EntityType_x003E_k__BackingField>
<d6p1:description i:nil="true" />
<d6p1:editState>Changed</d6p1:editState>
<d6p1:id>0</d6p1:id>
<d6p1:isInit>true</d6p1:isInit>
<d6p1:key>0</d6p1:key>
<d6p1:name i:nil="true" />
<d6p1:timeStamp>2009-09-10T21:20:29.67825Z</d6p1:timeStamp>
<d6p1:_x003C_DataType_x003E_k__BackingField xmlns:d7p1="http://schemas.datacontract.org/2004/07/System" i:type="d7p1:RuntimeType" />
</d5p1:Filter>

Argh. Problem resulting from missing [DataContact] attribute. I though the DCS recognized the [Serializable] attribute.