Null pointer error

[matklad]

$ cat main.djot 
write!(f, "({}")?

$ lua ./bin/main.lua main.djot
lua: ./djot/ast.lua:247: attempt to get length of local 'cs' (a nil value)
stack traceback:
    ./djot/ast.lua:247: in function 'insert_attributes_from_nodes'
    ./djot/ast.lua:563: in function 'get_node'
    ./djot/ast.lua:355: in function 'get_node'
    ./djot/ast.lua:355: in function 'get_node'
    ./djot/ast.lua:705: in function 'to_ast'
    ./djot.lua:53: in function 'build_ast'
    ./bin/main.lua:97: in main chunk
    [C]: in ?

[jgm]

Also just hi{} (empty inline attributes).