Changing text box control sources via combo box values

[cifdtruckie]

I want the user-chosen value in a combo box to determine the control source for a text box. I thought it would be as simple as:

Private Sub TextBox1_Change()
If AB1 = "January" Then
TextBox1.ControlSource = C5
ElseIf AB1 = "February" Then TextBox1.ControlSource = C6
ElseIf AB1 = "March" Then TextBox1.ControlSource = C7
ElseIf AB1 = "April" Then TextBox1.ControlSource = C8
End If
End Sub

Where AB1 is the control source for the combo box. Apparently it's not that simple - I'm not getting any debugging errors, but it just doesn't seem to work.
Any suggestions?

[Stuie]

is C5 to C8 referring to a range on a worksheet if so then this shoud work,

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[cifdtruckie]

Still not working - not getting an error, but the text box value is still not showing up.