The shape of the input to "Flatten" is not fully defined

[NanYoMy]

hi, i got this error info:
Traceback (most recent call last):
File "main.py", line 87, in
model = vgg_bn()
File "main.py", line 68, in vgg_bn
model.add(Flatten())
File "/home/nanyomy/anaconda2/envs/keras_env/lib/python2.7/site-packages/keras/models.py", line 469, in add
output_tensor = layer(self.outputs[0])
File "/home/nanyomy/anaconda2/envs/keras_env/lib/python2.7/site-packages/keras/engine/topology.py", line 615, in call
output_shape = self.compute_output_shape(input_shape)
File "/home/nanyomy/anaconda2/envs/keras_env/lib/python2.7/site-packages/keras/layers/core.py", line 477, in compute_output_shape
'(got ' + str(input_shape[1:]) + '. '
ValueError: The shape of the input to "Flatten" is not fully defined (got (0, 7, 128). Make sure to pass a complete "input_shape" or "batch_input_shape" argument to the first layer in your model.

did this problem caused by the different version of keras?