Why can't adding items into a new group?

I want to group circle with radius =5 from the existing group called "c", but when excute
"addGroup.AppendItems addEnt"
an error message is showed
Err : Method 'AppendItems' of object 'IAcadGroup' failed

Why I such method failed? Where is the problem?

Public Sub TestGroup()
Dim objGroup As AcadGroup
Dim addGroup As AcadGroup
Dim addEnt() As AcadEntity
Dim objEnt As AcadEntity
Dim showEnt As AcadEntity
Dim strName As String
Dim addName As String
Dim ct As Integer
Dim count As Integer
Dim varCen As Variant

On Error Resume Next
With ThisDrawing.Utility

''get the existing group
strName = "c"
Set objGroup = ThisDrawing.Groups.Add(strName)

''add new group
addName = "new"
Set addGroup = ThisDrawing.Groups.Add(addName)

''resize the entity aray
count = 0
For ct = 0 To objGroup.count - 1
Set objEnt = objGroup.Item(ct)
If objEnt.Radius = 5 Then
count = count + 1
End If
Next ct
.Prompt vbCrLf & "count = " & count
ReDim addEnt(count)

''copy entities to entity array
count = -1
For ct = 0 To objGroup.count - 1
Set objEnt = objGroup.Item(ct)

If objEnt.Radius = 5 Then
count = count + 1
Set addEnt(count) = objEnt
If Err Then .Prompt vbCrLf & "Err set : " & Err.Description
.Prompt vbCrLf & "#" & ct & " radius = " & objEnt.Radius
End If
Next ct

.Prompt vbCrLf & "now in addEnt:"
For ct = 0 To count
varCen = addEnt(ct).Center
.Prompt vbCrLf & "#" & ct & " radius = " & addEnt(ct).Radius & _
" Center at " & varCen(0) & "," & varCen(1)
Next ct

'' group circle
addGroup.AppendItems addEnt
If Err Then .Prompt vbCrLf & "Err : " & Err.Description

.Prompt vbCrLf & "Now in addEnt have: "
For ct = 0 To count
Set showEnt = addGroup.Item(ct)
.Prompt vbCrLf & "#" & ct & " radius = " & showEnt.Radius
Next ct

End With

End Sub