Follow the 3 steps in the screen capture.  The configuration uses a template that treats each of the attribute names within the link-entity as a lookup and is unable to distinguish whether it is a lookup or not so it applies the template to all of the attributes. You will have to manipulate the names and types of the fields. In the example below let assume I have agfou column that has the correct value in it is. After changing the name and datatype it will not error in downstream.

In the example, you will go to Rest Source -> Columns Tab -> Advance and then change the name and datatype for columns usr-firstname.value and usr-fullname.value to something like ext_usr-firstname and ext_usr-fullname. See below screen capture for reference. 

<fetch>

  <entity name="contact">

    <attribute name="contactid" />

    <attribute name="agf_username" />

    <attribute name="statecode" />

    <filter type="and">

      <condition attribute="statecode" operator="eq" value="0" />

      <condition attribute="agf_username" operator="not-null" />

    </filter>

    <link-entity name="account" from="accountid" to="parentcustomerid" link-type="inner" alias="acc">

      <attribute name="agf_agfou" />

<attribute name="accountid" />

      <filter type="and">

        <condition attribute="agf_agfdistrict" operator="eq" value="1" />

      </filter>

    </link-entity>

  </entity>

</fetch>