Beam join with double stream join key

hello, in my case, I wan't to join  double stream, while my join key is a customer-defined Class(DataWithMeta), but exected failed! Below is the result! I don't know why 

Re: Beam join with double stream join key

Hi, It looks like you may have tried to attach an image or something, but it did not come through the mailing list. Can you please try again? This is what we see: https://lists.apache.org/thread.html/f4a1ce5291428a70ecd54d3eefff56daf2f32b7a558f575eddc3729e@%3Cdev.beam.apache.org%3E Dan On Tue,