Cannot unprotect Excel worksheet in VBA (Run-time error 32809)

The below code has worked on this machine previously. I worked from home yesterday on a machine which hasn't had the December 2014 Office updates removed.

Private Sub DurationCombo_Change() 'Active if duration combo box value is changed

ActiveWorkbook.Worksheets("SUMMARY").Unprotect
ActiveWorkbook.Worksheets("RATES").Unprotect
ActiveWorkbook.Worksheets("MAIN SCOPE").Unprotect

'Dim sheetsCount As Integer
'Dim i As Integer
'    'Add number of sheets to integer
'    sheetsCount = ActiveWorkbook.Worksheets.Count
'    For i = 1 To sheetsCount - 2
'        ActiveWorkbook.Worksheets(i).Unprotect 'Unprotect first three sheets
'        Application.ScreenUpdating = False 'To prevent flickering of worksheets in for loop
'    Next i

'Select case here: omitted.

    For i = 1 To sheetsCount - 2
        ActiveWorkbook.Worksheets(i).Protect 'Protect first three sheets
    Next i
    Application.ScreenUpdating = True 'Restore initial setting

End Sub

I cannot run this code. It craps out on the line:

ActiveWorkbook.Worksheets("MAIN SCOPE").Unprotect

I get the same error when running the little for loop which you can see commented out. Any suggestions? The sheet exists. Unhidden. Not password-protected.

Thank you!