尝试在Keras中将XCeption模型用作图层时，Keras中出现异常

from keras.applications.xception import Xception
from keras.layers import Input, Flatten
from keras.models import Model

base_model = Xception(weights='imagenet', include_top=False, input_shape=(224, 224, 3))

input_tensor = Input(shape=(224, 224, 3))
x = base_model(input_tensor)
x = Flatten()(x)
output_tensor = Dense(num_classes, activation='softmax')(x)  # 这里的num_classes是分类的类别数
model = Model(inputs=input_tensor, outputs=output_tensor)

model.compile(optimizer='adam', loss='categorical_crossentropy', metrics=['accuracy'])
model.fit(x_train, y_train, batch_size=32, epochs=10, validation_data=(x_val, y_val))