Help urgently needed for this:

Sub x()
Dim a As String
Dim b As Variant
ReDim b(1 To 2, 1 To 1)
b(1, 1) = 1
b(2, 1) = 2
a = "Sum"

'This Works fine: "3"
MsgBox WorksheetFunction.Sum(b)

'This does not work: "WorksheetFunction.Sum(b)"
'I need to "force" an evaluation of the expression "WorksheetFunction.Sum(b)"
MsgBox "WorksheetFunction." & a & "(b)"
End Sub

Thanks a lot in advance !