WorksheetFunction.Average --> Function not working in this code --> Whats wrong? Also tried Application.WorksheetFunction.Average...

Private Sub T_01_Change()
T_01.Value = Replace(T_01.Value, ",", ".") ' Heizwärmebedarf Wärmeerzeuger 1 2015
    
T_13.Value = WorksheetFunction.Average(Val(T_01.Value), (T_02.Value), (T_03.Value), (T_04.Value))
'    ' Berechnung Mittelwert Heizwärmebedarf Wärmeerzeuger 1
    
End Sub

When I type 
T_13.Value = (Val(T_01.Value) + Val(T_02.Value) + Val(T_03.Value) + Val(T_04.Value)) / 4
--> it works