[求助]：为什么第二次修改有问题

gps

我想修改多段线高程,代码如下
Sub cc()
Dim pl As Acad3DPolyline
Dim returnobj As AcadEntity
Dim basepnt As Variant
Dim pl1 As AcadLWPolyline
Dim pt1(0 To 9998) As Double
ActiveDocument.Utility.GetEntity returnobj, basepnt, "选择多段线:"
k = 0
gc = InputBox("修改后高程:", gc)
retCoord = returnobj.Coordinates
For Number = LBound(retCoord) To UBound(retCoord) + 2 - 2 Step 2
pt1(k) = retCoord(Number)
pt1(k + 1) = retCoord(Number + 1)
pt1(k + 2) = gc
k = k + 3
Next
returnobj.Delete
Close #1
10 pt1(k) = pt1(k - 3)
pt1(k + 1) = pt1(k - 2)
pt1(k + 2) = gc
k = k + 3
If k < 9999 Then GoTo 10
Set pl = ThisDrawing.ModelSpace.Add3DPoly(pt1)
End Sub
第一次运行正确,但再次修改刚才的多段线便提示如图错误,为什么