ValueError：Shape（？，21，2）在tf.SparseTensor中的等级必须为2

Question

我的代码：

def f(x):
    import tensorflow as tf
    print('x.shape={0}'.format(x.shape))
    idx = K.cast(x[:, :, 0:2]*15.5+15.5, "int64")
    print('idx.shape={0}'.format(idx.shape))
    st_z = tf.SparseTensor(idx, values=0.0, dense_shape=[K.shape(x)[0], 32, 32])

输出：

x.shape=(?, 21, 3)
idx.shape=(?, 21, 2)

错误：

ValueError: Shape (?, 21, 2) must have rank 2

如何解决此问题？谢谢