导入relu激活时出错

Question

笔记本电脑环境: Python3

if K.image_data_format() == 'channels_first':
    input_shape = (3, img_rows, img_cols)
else:
    input_shape = (img_rows, img_cols, 3)

model = Sequential()    
model.add(Conv2D(32, (3, 3), input_shape=input_shape)
model.add(Activation('relu'))
model.add(MaxPooling2D(pool_size=(2, 2)))

model.add(Conv2D(32, (3, 3)))
model.add(Activation('relu'))
model.add(MaxPooling2D(pool_size=(2, 2)))

model.add(Conv2D(32, (3, 3)))
model.add(Activation('relu'))
model.add(MaxPooling2D(pool_size=(2, 2)))

错误：

File "<ipython-input-6-50fd40b32811>", line 21
    model.add(Activation('relu'))
        ^
SyntaxError: invalid syntax

我检查了导入relu的keras文档，它完全相同。这里出了什么问题？