Python export_graphviz函数，索引器错误：列表索引超出范围

在python中运行决策树时，除了导出树的图形外，其他所有操作都非常有效：

feature_cols = ['buildings_age','cell_id', 'pois:total', 'vertical_density',
                'Buildings:total', 'buildings_age:diversity',
                'ThirdPlaces:oa_count', 'pois:diversity', 'Roads:diversity',
                'Buildings:diversity']
X = data[feature_cols] # Features
y = data.most_present_age # Target variable
# Split dataset into training set and test set
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.3, random_state=1) # 70% training and 30% test
# Create Decision Tree classifier object
clf = DecisionTreeClassifier(criterion='gini', max_depth=3)
# Train Decision Tree Classifier
clf = clf.fit(X_train,y_train)
#Predict the response for test dataset
y_pred = clf.predict(X_test)
print("Accuracy:",metrics.accuracy_score(y_test, y_pred))
dot_data = StringIO()
export_graphviz(clf, out_file=dot_data,  
                filled=True, rounded=True,
                special_characters=True,
                feature_names = feature_cols,class_names=[])
graph = pydotplus.graph_from_dot_data(dot_data.getvalue())  
graph.write_png('tree.png')
Image(graph.create_png())

这会给出错误消息：

class_name = self.class_names[np.argmax(value)]
IndexError: list index out of range

这显示来自导入的函数export_graphviz 类名为['0'，'1'] 我很确定这是正确的，但是功能列表的长度也是错误的吗？或者是我没听说过的类名？ 非常感谢您的帮助，谢谢