AttributeError: 'tuple' object has no attribute 'layer'

[WAI-f]

I installed keras 2.2.4 and tensorflow 1.14, if I tried like this:
input = tf.ones((1, 256, 256, 32, 3), dtype=tf.float32)
x = DepthwiseConv3D(kernel_size=(3, 3, 3), depth_multiplier=2)(input)
the result is ok.
but when I tried like:
def conv_separable(net, width, ksize=ksize, s=1, d=1):
ret = DepthwiseConv3D(kernel_size=(ksize,ksize,ksize),
strides=(s, s, s),
dilation_rate=(d, d, d),
padding='SAME',
activation='relu'
)(net)
ret = conv(ret, width, 1, 1, 1)
return ret
it will record error:
inbound_layers = nest.map_structure(lambda t: t._keras_history.layer,
AttributeError: 'tuple' object has no attribute 'layer'
I don't know why.