Undescriptive runtime error when well-formedness with a symbol table entry fails

[xFrednet]

I have a well-formedness similar to this:

inline const trieste::wf::Wellformed flatten =
    (Top <<= File)
    | (File <<= (Label | Print)++)
    | (Label <<= Ident)[Ident];
    ;

Notice that the Label node has an identifier which should be added to the symbol table.

A rewrite rule returned something like this:

return Seq << (Print ^ " (True)")
           << (Label ^ "else:") << (Ident ^ else_label)
           << (Print ^ "(False"));

The label construction is incorrect, as I was missing parentheses to add the Ident as a Child of Label.

I expected the well-formedness to fail. Instead, I got the following undescriptive runtime error:

terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 0) >= this->size() (which is 0)
Aborted (core dumped)

Ideally, it should just throw the usual error about the well-formedness error