Error on the size of some tensors

Exception has occurred: RuntimeError
The size of tensor a (512) must match the size of tensor b (256) at non-singleton dimension 3
  File "C:\Users\chouaib\Documents\Markaiter\develop\tnt-kid\tnt_kid\model\transformer.py", line 331, in _attn
    w = w * b + -1e10 * (1 - b)

Any help ?

Thankx in Advance.