CPU version error

[jqjjcp]

Dear Arturus:
I just use tf.nn.dynamic_rnn+GRUCell to place cudnn but come cross that error:
"Input tensor 'rnn/while/Exit_3:0' enters the loop with shape (283, 128), but has shape (?, 128) after one iteration. To allow the shape to vary across iterations, use the shape_invariants argument of tf.while_loop to specify a less-specific shape."
when my code run the loop"_, _, _, targets_ta, outputs_ta = tf.while_loop(cond_fn, loop_fn, loop_init)"
How can i solve it?