invalid array name

I write
calculator = vtk.vtkArrayCalculator() calculator.SetInputConnection(cellToPoint.GetOutputPort()) calculator.SetAttributeType(vtk.vtkDataObject.POINT ) print(calculator.GetNumberOfScalarArrays()) calculator.AddScalarArrayName("VelocityX",0)
and I think it’s no problem,but the command windows get me a error message invalid array name "VelocityX",I do not konw where is wrong ,who can help me.Thanks.By the way,the cellToPoint.GetOutputPort() output type is vtkPolyData