How to add data validation, only allow numbers in cell vb.net /EPPLUS

I have tried to do the following but it doesn't work

Dim worksheet As ExcelWorksheet = package.Workbook.Worksheets.Add("Users")

            Dim formula As String = "=ISNUMBER(A2)"
            Dim validation = worksheet.DataValidations.AddCustomValidation("A:A")
            validation.Formula.ExcelFormula = formula