code: def _check_y_exog(self, y, exog, null_allowed=False):
          
          y, exog = super(BaseExogTransformer, self)._check_y_exog(y, exog)
          if exog is None and not null_allowed:
              raise ValueError("exog must be non-None for exogenous "
                               "transformers")
          return y, exog
docs: Check the endog and exog arrays