Evolutionary solver + excel run-time error 1004 reference is not valid

[samerwakim]

Hi everyone,

I am trying to run an optimization using excel 2016, however when choosing evolutionary solver the following error occurs:
"excel run-time error 1004 reference is not valid"
debuging this error takes me to A line in my small goal seek VBA code:
_________
Private Sub Worksheet_Activate()

End Sub
Sub Worksheet_Calculate()
Application.EnableEvents = False
Range("U51").GoalSeek Goal:=0, ChangingCell:=Range("R6")
Application.EnableEvents = True
End Sub


Private Sub Worksheet_Change(ByVal Target As Range)

End Sub
________

Running the optimization using the other two solver poses no problems,

Pleass does anyone know whats the problem?
Thanks in advance