TextBox Setfocus Unexpected Call

I've got a user form with 12 textbox, one for each month a total textbox number 13 and a 14th text box (TB28) to enter $'s in.

AfterUpdate in each textbox 1-12 posts its value to a worksheet all values are summed in a =sum range which in turn populates textbox 13. When TB13.text = 100.00 it sucessfully calls a number of routines but what I can't get it to do is set focus on TB28.

Private Sub TextBox13_Change()
If TextBox13.Text = "100.00" Then
With FormPhasedAmt.TextBox28
.SelStart = 0
.SelLength = Len(TextBox28.Text)
.SetFocus
End With
End If
End Sub

It returns a Run time error Unexpected call to method or property access highlighting .SetFocus. The rest of the code works OK. Is it because other textbox on the same user form have text highlighted because of the tab order?