for fold, (train, test) in enumerate(kf.split(X)):
            if len(np.unique(y[train])) != len(self.classes_):
                continue
            fold_candidate = clone(self.base_estimator).fit(self.X_[train], self.y_[train])
            msei = self.msei(fold_candidate, self.X_[test], self.y_[test])