problem was somewhere else

IF "Sales Invoice Line".Type = "Sales Invoice Line".Type::Artikel THEN BEGIN
  lr_CrRef.SETRANGE("Item No.","Sales Invoice Line"."No.");
  lr_CrRef.SETRANGE("Variant Code", "Sales Invoice Line"."Variant Code");
  lr_CrRef.SETRANGE("Unit of Measure","Sales Invoice Line"."Unit of Measure");
  lr_CrRef.SETRANGE("Cross-Reference Type",lr_CrRef."Cross-Reference Type"::Klant);
  lr_CrRef.SETRANGE("Cross-Reference Type No.","Sales Invoice Header"."Sell-to Customer No.");
  IF lr_CrRef.FINDFIRST THEN BEGIN
    "Sales Invoice Line"."Sidec No." := lr_CrRef."Cross-Reference No.";
    "Sales Invoice Line".Description := lr_CrRef.Description;
    "Sales Invoice Line"."Description 2" := '';
  END;
END;