ظهور الخطأ ValueError: Error when checking model input في كيراس Keras

قمت ببناء النموذج التالي في كيراس، لكن يطهر لي الخطأ التالي:

backend.set_image_dim_ordering('tf')
np.random.seed(7)
x_train = x_train.astype('float32')/255.0
x_test = x_test.astype('float32')/255.0
y_train = np_utils.to_categorical(y_train)
y_test = np_utils.to_categorical(y_test)
model = Sequential()
model.add(Convolution2D(16, 3, 3,, W_constraint=maxnorm(3),
                            input_shape=(3, 32, 32), border_mode='same',
                            activation='tanh'))
model.add(Dropout(0.3))
model.add(Convolution2D(32, 3, 3,, W_constraint=maxnorm(3),
                            activation='tanh', border_mode='same'))
model.add(Flatten())
model.add(Dense(400, activation='relu', W_constraint=maxnorm(3)))
model.add(Dropout(0.4))
model.add(Dense(y_test.shape[1], activation='softmax'))
model.compile(loss='categorical_crossentropy', optimizer="adam", metrics=['acc'])
model.fit(x_train, y_train,batch_size=64, validation_data=(x_test, y_test), nb_epoch=30)
model.evaluate(x_test, y_test)
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ValueError: Error when checking model input: expected convolution2d_input_1 to have shape (None, 3, 32, 32) but got array with shape (50000, 32, 32, 3)