python：機械学習の結果（正解/予測）をcompareするとエラーが出てしまいます。。

以下のコードを実行すると、なぜかエラーが出てしまいます。
（ValueError: Length mismatch: Expected axis has 1 elements, new values have 2 elements）
お詳しい方、ご指導をお願いいたします。

y_pred_std = clf.predict(X_pred_std).flatten()
print(y_pred_std[:50])
print(y_test[:50])
compare = pd.DataFrame(np.array([y_test,y_pred_std]).T)
compare.columns = ['正解','予測値']
compare[:10]

[24.86740433 22.30359119 31.71576534 32.52761891 31.63410178 25.88693233
 21.07515894 17.46155088 16.60679608 18.39296644 15.80763186 19.70884437
 22.47479312 18.3803265  17.96495459 18.07257959 20.29210955 16.91049371
 19.50175167 17.49186267 14.846172   17.12078862 15.97273836 15.12065853
 15.98052488 14.50042306 15.81739873 14.48911213 17.67135571 19.09828343
 13.45324495 16.81108266 12.51722343 15.12857997 13.08153136 20.93689338
 20.71315586 20.42861738 22.30286696 29.00888868 35.48719263 30.65809822
 24.63380414 24.53509624 22.18019432 20.0259868  20.73546208 18.39766905
 15.09929938 18.72251231]
[20.3 42.8 23.1 27.5 37.9  8.3 14.5 22.8 20.  31.2 20.4 22.6 23.8 17.9
 10.5 16.  14.4 23.  35.1 22.9 21.6 20.1  7.4 18.9 48.8 11.7  8.5 24.8
 23.6 50.  13.4 19.6 32.9 22.2 27.9 18.8 50.  33.  24.3 21.  24.2 23.9
 24.1 18.3 22.6 23.  16.2 12.   5.6 23.5]

以下、エラーメッセージ

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-118-a381b5ded83a> in <module>
      4 compare = pd.DataFrame(np.array([y_test,pred]).T)
      5 compare = pd.DataFrame(np.array([y_test,y_pred_std]).T)
----> 6 compare.columns = ['正解','予測値']
      7 compare[:10]

C:\python\anaconda\pgm\lib\site-packages\pandas\core\generic.py in __setattr__(self, name, value)
   5078         try:
   5079             object.__getattribute__(self, name)
-> 5080             return object.__setattr__(self, name, value)
   5081         except AttributeError:
   5082             pass

pandas/_libs/properties.pyx in pandas._libs.properties.AxisProperty.__set__()

C:\python\anaconda\pgm\lib\site-packages\pandas\core\generic.py in _set_axis(self, axis, labels)
    636 
    637     def _set_axis(self, axis, labels):
--> 638         self._data.set_axis(axis, labels)
    639         self._clear_item_cache()
    640 

C:\python\anaconda\pgm\lib\site-packages\pandas\core\internals\managers.py in set_axis(self, axis, new_labels)
    153             raise ValueError(
    154                 'Length mismatch: Expected axis has {old} elements, new '
--> 155                 'values have {new} elements'.format(old=old_len, new=new_len))
    156 
    157         self.axes[axis] = new_labels

ValueError: Length mismatch: Expected axis has 1 elements, new values have 2 elements