Keras ValueError:layer sequential_3的输入0与layer:输入形状的预期轴-1不兼容

我的代码给出了一个valueError。有人知道为什么吗

ValueError:layer sequential_3的输入0与layer:输入形状的预期轴-1不兼容

这是我的代码：

import keras
from keras.models import Sequential
from keras.layers import Dense
classifier = Sequential()
classifier.add(Dense(4, kernel_initializer='uniform', activation = 'relu', input_dim = 9))
classifier.add(Dense(4, kernel_initializer='uniform', activation = 'relu'))
classifier.add(Dense(1, kernel_initializer='uniform', activation = 'sigmoid'))
classifier.compile(optimizer = 'adam', loss = 'binary_crossentropy', metrics = ['accuracy'])
classifier.fit(X_train, y_train, batch_size = 5, epochs = 100)