When a BlockLinearOperator with at least one block that is not a ZeroOperator is summed with a ZeroOperator, the following error appears:

'ZeroOperator' object has no attribute 'set_backend'
  File "/psydac/psydac/linalg/block.py", line 1257, in set_backend
    b.set_backend(backend)

With the inverse order (summing ZeroOperator + BlockLinearOperator) there is no error.