delphi里控制Word问题，VBA高手请进(50分)

With Wordapplication1 Do
Begin
Unit_ := 'wdLine';
Count := '1';
Extend:= 'wdMove';
Selection.MoveDown(Unit_, Count, Extend);
End;
为什么执行到Selection.MoveDown(Unit_, Count, Extend)就老是出错说类型不匹配哪？？

var Unit_,Count,Extendlevariant;

With Wordapplication1 Do
Begin
Unit_ := wdLine;
Count := 1;
Extend:= wdMove;
Selection.MoveDown(Unit_, Count, Extend);
End;

接受答案了.