# Buggy code
def visit_set_expr(self, expr: SetExpr) -> None:
    for item in expr.items:
        item.accept(self)


# Traceback (AttributeError)
Traceback (most recent call last):
File "/home/dominik/.virtualenvs/mypy/bin/mypy", line 6, in <module>
main(__file__)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/main.py", line 54, in main
res = type_check_only(sources, bin_dir, options)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/main.py", line 102, in type_check_only
python_path=options.python_path)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/build.py", line 211, in build
dispatch(sources, manager)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/build.py", line 1336, in dispatch
process_graph(graph, manager)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/build.py", line 1467, in process_graph
process_stale_scc(graph, scc)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/build.py", line 1497, in process_stale_scc
graph[id].type_check()
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/build.py", line 1316, in type_check
manager.type_checker.visit_file(self.tree, self.xpath)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/checker.py", line 426, in visit_file
self.accept(d)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/checker.py", line 476, in accept
typ = node.accept(self)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/nodes.py", line 729, in accept
return visitor.visit_expression_stmt(self)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/checker.py", line 1582, in visit_expression_stmt
self.accept(s.expr)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/checker.py", line 476, in accept
typ = node.accept(self)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/nodes.py", line 1434, in accept
return visitor.visit_list_expr(self)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/checker.py", line 2122, in visit_list_expr
return self.expr_checker.visit_list_expr(e)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/checkexpr.py", line 1251, in visit_list_expr
e)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/checkexpr.py", line 1270, in check_list_or_set_expr
[nodes.ARG_POS] * len(items), context)[0]
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/checkexpr.py", line 242, in check_call
messages=arg_messages)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/checkexpr.py", line 661, in check_argument_types
actual + 1, i + 1, callee, context, messages)
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/checkexpr.py", line 686, in check_arg
elif not is_subtype(caller_type, callee_type):
File "/home/dominik/.virtualenvs/mypy/lib/python3.5/site-packages/mypy/subtypes.py", line 49, in is_subtype
return left.accept(SubtypeVisitor(right, type_parameter_checker))
AttributeError: 'NoneType' object has no attribute 'accept'

# Fixed code
def visit_set_expr(self, expr: SetExpr) -> None:
    for item in expr.items:
        if isinstance(item, StarExpr):
            item.valid = True
        item.accept(self)