我收到以下错误

ValueError: as_list() is not defined on an unknown TensorShape.

我的代码如下所示

# define input
X_input = Input(shape=(n_features, n_channels))
   
# define features extractor model
features = Lambda(
    function=extract_features_lambda,
    output_shape=(None,)
)(X_input)

# CNN block
X = Reshape((n_steps, n_length, n_channels))(X_input)
    
X = TimeDistributed(
    Conv1D(filters=32, kernel_size=5, activation='relu'),
    input_shape=(None, n_length, n_features)
)(X)
    
X = TimeDistributed(
    Conv1D(filters=64, kernel_size=7, activation='relu')
)(X)

X = TimeDistributed(
    Conv1D(filters=32, kernel_size=5, activation='relu')
)(X)

X = TimeDistributed(Dropout(0.5))(X)
X = TimeDistributed(MaxPooling1D(pool_size=2))(X)
X = Flatten()(X)
    
# merge the 2 features
X = Concatenate()([features, X])

Lambda图层包含了一个自定义的特征提取器函数。这个函数计算一些特征并返回一个numpy数组。

def extract_features(X):
    features = np.zeros(29, X.shape[1])

    # compute the features ...

    return features.flatten()

def extract_features_lambda(X):
    features = tf.py_function(
        extract_features,
        [X],
        tf.float32
    )
    
    features.set_shape = ((None, 29*12))
    
    return features

我做错了什么？