I'm new to userforms and can't find a solution.

I'm wanting to use this rough-draft macro and userform to let get me
choose a worksheet of a workbook from a combo box, and then reuse it
again to let me choose a worksheet from another workbook in the combo box.

It seems to work ok the first time around, but the second time the combo
box wants to display the same worksheets from the first time around, and
then goes to an error.

Can someone tell me what's wrong?
Thanks.

==================
Here's the macro:
==================

Sub test()

Dim MainWB As String
Dim wbDestination As String
Dim wbReference As String
Dim CellValue As String
Dim tester As String

MainWB = ActiveWorkbook.Name
wbDestination = Sheets("Sheet1").Range("B4").Value
wbReference = Sheets("Sheet1").Range("B4").Value

Windows(MainWB).Activate
CellValue = Sheets("Sheet1").Range("B4").Value
CellLocation = "B4"

If CellValue = "" Then
Windows("1stWorkbook.xls").Activate
frmDropIt.Show
tester = frmDropIt.ComboBox1.Value
Windows(MainWB).Activate
Range(CellLocation).Value = tester
Else
'Proceed to make sure the worksheet is the primary choice
End If

Windows(MainWB).Activate
CellValue = Sheets("Sheet1").Range("B7").Value
CellLocation = "B7"

If CellValue = "" Then
Windows("2ndWorkbook.xls").Activate
frmDropIt.Show
tester = frmDropIt.ComboBox1.Value
Windows(MainWB).Activate
Range(CellLocation).Value = tester
Else
'Proceed to make sure the worksheet is the primary choice
End If
End Sub

==================
Here's the Form:
==================

Private Sub cmdOK_Click()
'Dim ReturnThis As String

If Me.ComboBox1.Value = "" Then
Unload Me
Else
Worksheets(Me.ComboBox1.Value).Activate
'MsgBox "You chose: " & Me.ComboBox1.Value
Unload Me
End If
End Sub

Private Sub ComboBox1_Change()
'No code here
End Sub

Private Sub cmdCancel_Click()
Unload Me
End Sub

Private Sub UserForm_Initialize()
Dim i As Long

For i = 1 To Worksheets.Count
Me.ComboBox1.AddItem Worksheets(i).Name
Next
ComboBox1.Value = Worksheets(1).Name
End Sub