C# – SerializationException from System.Web.UI.Page.SaveAllState() on a member marked with NonSerializedAttribute

I get a serialization error on a page containing a custom control. the control has a member (dataContext) of a type (EntityContext) that is non-serializable, and marked as such.

this is the error:

Type 'Entities.EntityContext' in
Assembly '…' is not marked as
serializable.

[SerializationException: Type
'…Entities.EntityContext'
in Assembly '…' is
not marked as serializable.]
System.Runtime.Serialization.FormatterServices.InternalGetSerializableMembers(RuntimeType
type) +7733643
System.Runtime.Serialization.FormatterServices.GetSerializableMembers(Type
type, StreamingContext context) +258
System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitMemberInfo()
+111 System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitSerialize(Object
obj, ISurrogateSelector
surrogateSelector, StreamingContext
context, SerObjectInfoInit
serObjectInfoInit, IFormatterConverter
converter, ObjectWriter objectWriter)
+161 System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.Serialize(Object
obj, ISurrogateSelector
surrogateSelector, StreamingContext
context, SerObjectInfoInit
serObjectInfoInit, IFormatterConverter
converter, ObjectWriter objectWriter)
+51 System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Serialize(Object
graph, Header[] inHeaders,
__BinaryWriter serWriter, Boolean fCheck) +410
System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream
serializationStream, Object graph,
Header[] headers, Boolean fCheck) +134
System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream
serializationStream, Object graph) +13
System.Web.UI.ObjectStateFormatter.SerializeValue(SerializerBinaryWriter
writer, Object value) +4966

[ArgumentException: Error serializing
value
'…Entities.EntityContext'
of type
'…Entities.EntityContext.']
System.Web.UI.ObjectStateFormatter.SerializeValue(SerializerBinaryWriter
writer, Object value) +5425
System.Web.UI.ObjectStateFormatter.Serialize(Stream
outputStream, Object stateGraph) +163
System.Web.UI.ObjectStateFormatter.Serialize(Object
stateGraph) +99
System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Serialize(Object
state) +37
System.Web.UI.Util.SerializeWithAssert(IStateFormatter
formatter, Object stateGraph) +55
System.Web.UI.HiddenFieldPageStatePersister.Save()
+143 System.Web.UI.Page.SavePageStateToPersistenceMedium(Object
state) +190
System.Web.UI.Page.SaveAllState()
+1466 System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean
includeStagesAfterAsyncPoint) +5477

This is the control:

public class EntityDataSource : ObjectDataSource
{
    [NonSerialized] private EntityContext dataContext;

    /// <summary>
    /// Gets the data context. (This is used by the page at runtime.)
    /// </summary>
    /// <value>The data context.</value>
    // ReSharper disable MemberCanBePrivate.Global
    public EntityContext DataContext
    // ReSharper restore MemberCanBePrivate.Global
    {
        get { return dataContext; }
    }

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);
        dataContext = new EntityContext(SessionProvider.GetContext());
    }

    /// <summary>
    /// Viewstate is not implemented. This value allways return <c>false</c>.
    /// </summary>
    /// <exception cref="NotSupportedException">Exception is thrown when setting this value.</exception>
    [Browsable(false)]
    public override bool EnableViewState
    {
        get
        {
            return false;
        }
        set
        {
            //Throws exception if value is true.
            if (value)
            {
                throw new NotSupportedException("Viewstate is not enabled on this control.");
            }
        }
    }

    public override void Dispose()
    {
        if (DataContext != null)
        {
            DataContext.Dispose();
        }

        base.Dispose();
    }
}

It almost seems like the page doesn't honor the NonSerializedAttribute.
I have omitted the company and product names from the namespaces.