OPTIONAL is not supported in SPARQL UPDATE

[shinobu]

Attempting to use an OPTIONAL in a SPARQL UPDATE query, like this one:

DELETE {
    GRAPH <http://example.org> {
      ?s ?p ?o .
      ?o ?a ?b .
    }
  }
WHERE
    { GRAPH <http://example.org>
        { ?s  ?p  ?o .
          OPTIONAL { ?o ?a ?b . }
  } }

leads to an Error. The stacktrace looks like this:

argument of type 'NoneType' is not iterable
Traceback (most recent call last):
  File "quit/web/modules/endpoint.py", line 140, in sparql
  File "quit/core.py", line 421, in applyQueryOnCommit
  File "quit/core.py", line 90, in update
  File "site-packages/rdflib/graph.py", line 1099, in update
  File "quit/tools/processor.py", line 17, in update
  File "quit/tools/update.py", line 415, in evalUpdate
  File "quit/tools/update.py", line 250, in evalModify
  File "site-packages/rdflib/plugins/sparql/evaluate.py", line 226, in evalPart
  File "site-packages/rdflib/plugins/sparql/evaluate.py", line 111, in evalJoin
  File "site-packages/rdflib/plugins/sparql/evaluate.py", line 183, in evalGraph
  File "site-packages/rdflib/plugins/sparql/evaluate.py", line 134, in evalLeftJoin
  File "site-packages/rdflib/plugins/sparql/sparql.py", line 212, in forget
  File "site-packages/rdflib/plugins/sparql/sparql.py", line 165, in __init__
  File "site-packages/rdflib/plugins/sparql/sparql.py", line 107, in __init__
  File "site-packages/rdflib/plugins/sparql/sparql.py", line 214, in <genexpr>
TypeError: argument of type 'NoneType' is not iterable
::ffff:127.0.0.1 - - [16/May/2019 16:01:47] "POST /sparql HTTP/1.1" 400 -

@splattater was capable of reproducing the Error.

[white-gecko]

Does this also happen in the RDFlib?