code: def get_valid_error(x1, x2=-1):
      
      # just error
      if type(x2) == int and x2 == -1:
          try:    
              e = np.array(x1)
          except:
              raise ValueError('Impossible to convert series to a numpy array')        
      # two series
      else:
          try:
              x1 = np.array(x1)
              x2 = np.array(x2)
          except:
              raise ValueError('Impossible to convert one of series to a numpy array')
          if not len(x1) == len(x2):
              raise ValueError('The length of both series must agree.')
          e = x1 - x2
      return e
docs: Function that validates:
  
      * x1 is possible to convert to numpy array
  
      * x2 is possible to convert to numpy array (if exists)
  
      * x1 and x2 have the same length (if both exist)