ظهور الخطأ التالي 'ValueError: The last dimension of the inputs to `Dense` should be defined. Found 'None  أثناء بناء شبكة عصبية في Keras.

أقوم ببناء شبكة عصبية لمهمة NLP (مهمة تحليل مشاعر على بيانات imdb) لكن لا أعرف سبب الخطأ التالي:

from keras.datasets import imdb
from keras.layers import Embedding, SimpleRNN,Flatten,Dense
from keras.models import Sequential
(input_train, y_train), (input_test, y_test) = imdb.load_data(
num_words=10000)
print(len(input_train), 'train sequences')
print(len(input_test), 'test sequences')
################ نضيف###################
from keras.preprocessing import sequence
maxlen = 20
print('Pad sequences (samples x time)')
input_train = sequence.pad_sequences(input_train, maxlen=maxlen)
input_test = sequence.pad_sequences(input_test, maxlen=maxlen)
############# انتهى####################
print('input_train shape:', input_train.shape)
print('input_test shape:', input_test.shape)
from keras.layers import Dense
model = Sequential()
model.add(Embedding(10000, 16))
model.add(Flatten())
model.add(Dense(32, activation='relu'))
model.add(Dense(1, activation='sigmoid'))
model.compile(optimizer='rmsprop', loss='binary_crossentropy', metrics=['acc'])
history = model.fit(input_train, y_train,
epochs=2,
batch_size=32,
validation_split=0.2)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-10-08cd97ead789> in <module>
     19 model.add(Embedding(10000, 16))
     20 model.add(Flatten())
---> 21 model.add(Dense(32, activation='relu'))
     22 model.add(Dense(1, activation='sigmoid'))
     23 model.compile(optimizer='rmsprop', loss='binary_crossentropy', metrics=['acc'])

~\anaconda3\lib\site-packages\tensorflow\python\training\tracking\base.py in _method_wrapper(self, *args, **kwargs)
    515     self._self_setattr_tracking = False  # pylint: disable=protected-access
    516     try:
--> 517       result = method(self, *args, **kwargs)
    518     finally:
    519       self._self_setattr_tracking = previous_value  # pylint: disable=protected-access

~\anaconda3\lib\site-packages\tensorflow\python\keras\engine\sequential.py in add(self, layer)
    221       # If the model is being built continuously on top of an input layer:
    222       # refresh its output.
--> 223       output_tensor = layer(self.outputs[0])
    224       if len(nest.flatten(output_tensor)) != 1:
    225         raise ValueError(SINGLE_LAYER_OUTPUT_ERROR_MSG)

~\anaconda3\lib\site-packages\tensorflow\python\keras\engine\base_layer.py in __call__(self, *args, **kwargs)
    950     if _in_functional_construction_mode(self, inputs, args, kwargs, input_list):
    951       return self._functional_construction_call(inputs, args, kwargs,
--> 952                                                 input_list)
    953 
    954     # Maintains info about the `Layer.call` stack.

~\anaconda3\lib\site-packages\tensorflow\python\keras\engine\base_layer.py in _functional_construction_call(self, inputs, args, kwargs, input_list)
   1089         # Check input assumptions set after layer building, e.g. input shape.
   1090         outputs = self._keras_tensor_symbolic_call(
-> 1091             inputs, input_masks, args, kwargs)
   1092 
   1093         if outputs is None:

~\anaconda3\lib\site-packages\tensorflow\python\keras\engine\base_layer.py in _keras_tensor_symbolic_call(self, inputs, input_masks, args, kwargs)
    820       return nest.map_structure(keras_tensor.KerasTensor, output_signature)
    821     else:
--> 822       return self._infer_output_signature(inputs, args, kwargs, input_masks)
    823 
    824   def _infer_output_signature(self, inputs, args, kwargs, input_masks):

~\anaconda3\lib\site-packages\tensorflow\python\keras\engine\base_layer.py in _infer_output_signature(self, inputs, args, kwargs, input_masks)
    860           # overridden).
    861           # TODO(kaftan): do we maybe_build here, or have we already done it?
--> 862           self._maybe_build(inputs)
    863           outputs = call_fn(inputs, *args, **kwargs)
    864 

~\anaconda3\lib\site-packages\tensorflow\python\keras\engine\base_layer.py in _maybe_build(self, inputs)
   2708         # operations.
   2709         with tf_utils.maybe_init_scope(self):
-> 2710           self.build(input_shapes)  # pylint:disable=not-callable
   2711       # We must set also ensure that the layer is marked as built, and the build
   2712       # shape is stored since user defined build functions may not be calling

~\anaconda3\lib\site-packages\tensorflow\python\keras\layers\core.py in build(self, input_shape)
   1180     last_dim = tensor_shape.dimension_value(input_shape[-1])
   1181     if last_dim is None:
-> 1182       raise ValueError('The last dimension of the inputs to `Dense` '
   1183                        'should be defined. Found `None`.')
   1184     self.input_spec = InputSpec(min_ndim=2, axes={-1: last_dim})

ValueError: The last dimension of the inputs to `Dense` should be defined. Found `None`.