Passing NULL parameters to Stored Proc in Excel VBA

I'm trying to call a stored procedure that contains NULL parameters using Excel VBA.

The code works in SQL Server Management Studio -
Exec SP_AML_WIPS_Account_Data_Lookup '5670934',NULL,NULL

Here's my code vba code -

    Dim intDDA As Variant
    Dim intTIN As Variant
    Dim intPN As Variant
   
    intDDA = ThisWorkbook.Worksheets("WIPS Data Lookup").Range("E3").Value
    intTIN = ThisWorkbook.Worksheets("WIPS Data Lookup").Range("E5").Value
    intPN = ThisWorkbook.Worksheets("WIPS Data Lookup").Range("E7").Value
   
    If intDDA = 0 Then
       intDDA = Null
     Else
      intDDA = ThisWorkbook.Worksheets("WIPS Data Lookup").Range("E3").Value
    End If
   
    If intTIN = 0 Then
       intTIN = "Null"
    Else
      intTIN = ThisWorkbook.Worksheets("WIPS Data Lookup").Range("E5").Value
    End If
   
    If intPN = 0 Then
       intPN = "Null"
    Else
      intPN = ThisWorkbook.Worksheets("WIPS Data Lookup").Range("E7").Value
    End If
   
    With ActiveWorkbook.Connections("WIPs_Account_Data").OLEDBConnection
          .CommandText = "EXECUTE dbo.SP_AML_WIPS_Account_Data_Lookup '" & intDDA & "'" & "," & "'" & intTIN & "'" & "," & " '" & intPN & "'"   'Three parameters
    End With
    ActiveWorkbook.Connections("WIPs_Account_Data").Refresh

When I check the value of .CommantText, it reads

?.commandtext
EXECUTE dbo.SP_AML_WIPS_Account_Data_Lookup '6681365,'',''

I need it read as **EXECUTE dbo.SP_AML_WIPS_Account_Data_Lookup '6681365,NULL, NULL

I can't get NULL to be included in my EXECUTE statement, what am I missing, thanks for your help.