How to close an MDI child window

[clu82]

I have an MDI app that allows the user to open multiple client address child
forms. The user has a child form open that has client id "1001" and the
records associated data displayed. If the user chooses to open a new child
form and then try to select client id "1001" again, I would like to
automatically close this form and bring the other form that already had
client "1001" to the front.

With the code below, I can find the correct child form to bring to the
front, but when I try to close the "redundant" child form, I get an exception.

Private Sub txtClientNumber_LostFocus(ByVal sender As Object, ByVal e As
System.EventArgs) Handles txtClientNumber.LostFocus
Dim frm As frmDocument

For Each frm In Me.MdiParent.MdiChildren
'no need to check myself (the current form)
If (Me.Handle.ToInt32 <> frm.Handle.ToInt32) Then
'if the number just entered is already used on another form,
switch to that form
If (frm.txtClientNumber.Text = txtClientNumber.Text) Then
frm.TopMost() = True
Me.Close() '<- this causes an exception to be thrown
End If
End If
Next frm

End Sub

Does anyone have any ideas on how to accomplish this task: close a child
form from within the code of the child form without throwing an exception?

tia
clu82