If the document contains a simple field without run inside, the import process fails. 

      <w:fldSimple w:instr="page" w:dirty="true"/>

The same document with a run inside the field does not cause an error

      <w:fldSimple w:instr="page" w:dirty="true">
        <w:r>
          <w:t>1</w:t>
        </w:r>
      </w:fldSimple>