AttributeError: module 'keras.backend' has no attribute 'tf'

[a461925358]

Traceback (most recent call last):
File "F:/pycharm/bert_in_keras/relation_extract.py", line 240, in
k1v = Lambda(seq_gather)([t, k1])
File "D:\software\anoconda\envs\keras_bert\lib\site-packages\keras\engine\base_layer.py", line 451, in call
output = self.call(inputs, **kwargs)
File "D:\software\anoconda\envs\keras_bert\lib\site-packages\keras\layers\core.py", line 716, in call
return self.function(inputs, **arguments)
File "F:/pycharm/bert_in_keras/relation_extract.py", line 212, in seq_gather
return K.tf.gather_nd(seq, idxs)
AttributeError: module 'keras.backend' has no attribute 'tf'