AIF - Object reference not set to an instance of an object

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
   <s:Body>
      <s:Fault>
         <faultcode>s:Client</faultcode>
         <faultstring xml:lang="en-GB">Object reference not set to an instance of an object.</faultstring>
      </s:Fault>
   </s:Body>
</s:Envelope>


There could be many reason for this error message, but this post is just to share the reason we hit this error.

Below are the details:

• We have a document service for a query (a main datasource with 3 sub datasource)
• At one of the sub datasource, among the field list, one of the field is causing the error shown above
• If this field is removed from the query, it works fine
• Further investigation found that this field has a property of Mandatory = Yes, EDT of RefRecId
• In the respective table, there's a relation for this field linking to another table's RecId
  Eg: SubTable.FieldA == RelatedTable.RecId
• RelatedTable is an inherited table
• At the parent table for RelatedTable, is has a replacement key

This error only happen if this mandatory field is == 0.

We tried testing it against normal mandatory field (Eg. a text field), even if empty (not filled in), the error didn't occur, it only occur to field that's linked to another table (and when the above conditions are true)