For Each Loop - Involving LINQ Query When Reading Form Controls

I have a form with a tab control, containing a small number of tab pages. I'm writing a sub procedure to clear the controls within a groupbox for the selected tab. A selected tab will only ever contain ONE groupbox and within a groupbox will be command buttons, labels, text boxes and possibly a combobox. The first block of code below firstly finds the groupbox within the selected tab. The next part loops around all control type and if of type texbox clears the text.

Now hears the head scratcher/confusion it error's with :-
Unable to cast object of type 'System.Windows.Forms.Button' to type 'System.Windows.Forms.GroupBox
Which I don't understand as I cant see where Im converting objects simply looping through all controls and then ONLY if type of control is textbox then clear text.

'Loop around all controls in form, simplified that all controls
'are within a SINGLE groupbox and clear contents or reset to default value
Dim groupbox = (From grpbx In tabAdmin.SelectedTab.Controls _
Select grpbx = DirectCast(grpbx, GroupBox) _
Where TypeOf grpbx Is GroupBox)
[Code] .....