Unable to cast object of type System.Guid to type System.String

Hi there,

I am trying to override the address line 1 on Shipment screen by defaulting it to the User Defined Field selection (pointing to Customer table). However, when I got this error when I try to test it:

Below is my DAC as well as code for your reference:

DAC:

using System;
using PX.Data;
using PX.Data.BQL;
using PX.Objects.SO;

namespace PX.Objects.SO
{
  [Serializable]
  [PXCacheName("SOShipmentKvExt")]
  public class SOShipmentKvExt : IBqlTable
  {
    public abstract class recordID : BqlGuid.Field<recordID> { }
    [PXDBGuid(IsKey = true)]
    public Guid? RecordID { get; set; }

    public abstract class fieldName : BqlString.Field<fieldName> { }
    [PXDBString(50,IsKey = true)]
    [PXUIField(DisplayName ="Name")]
    public string FieldName { get; set; }

    public abstract class valueNumeric : BqlDecimal.Field<valueNumeric> { }
    [PXDBDecimal(8)]
    [PXUIField(DisplayName = "Value Numeric")]
    public decimal? ValueNumeric { get; set; }

    public abstract class valueDate : BqlDateTime.Field<valueDate> { }
    [PXDBDate]
    [PXUIField(DisplayName = "Value Date")]
    public DateTime? ValueDate { get; set; }

    public abstract class valueString : BqlString.Field<valueString> { }
    [PXDBString(256)]
    [PXUIField(DisplayName = "Value String")]
    public string ValueString { get; set; }

    public abstract class valueText : BqlString.Field<valueText> { }
    [PXDBString]
    [PXUIField(DisplayName = "Value Text")]
    public string ValueText { get; set; }
  }
}

Code:

using PX.CS.Contracts.Interfaces;
using PX.Data.ReferentialIntegrity.Attributes;
using PX.Data;
using PX.Objects.CR;
using PX.Objects.CS;
using PX.Objects.SO;
using PX.Objects;
using System.Collections.Generic;
using System;

namespace PX.Objects.SO
{
    // Acuminator disable once PX1011 InheritanceFromPXCacheExtension [Justification]
    // Acuminator disable once PX1016 ExtensionDoesNotDeclareIsActiveMethod extension should be constantly active
    // Acuminator disable once PX1026 UnderscoresInDacDeclaration [Justification]
    [PXNonInstantiatedExtension]
  public class SO_SOAddress_ExistingColumn : PXCacheExtension<PX.Objects.SO.SOAddress>
  {
      #region AddressLine1  
        [PXMergeAttributes(Method = MergeMethod.Append)]
        [PXDBScalar(typeof(Search2<SOAddress.addressLine1,
            InnerJoin<SOShipment, On<SOAddress.customerID, Equal<SOShipment.customerID>>,
            InnerJoin<SOShipmentKvExt, On <SOShipment.noteID, Equal<SOShipmentKvExt.recordID>>>>,
                Where<SOShipment.customerID, Equal<Current<SOShipmentKvExt.valueString>>>>))]
      public string AddressLine1 { get; set; }
      #endregion
  }

}

I suspect it is because the UDF is being treated as String while the Customer ID is treated as GUID, but I may be wrong. Any idea on how to tackle this issue?