Proposed resolution:

Add a new paragraph before 22.2.1.5.2, p5, and after the function declaration below

    result do_unshift(stateT& state,
    externT* to, externT* to_limit, externT*& to_next) const;

as follows:

    Requires: (to <= to_end) well defined and true; state initialized,
    if at the beginning of a sequence, or else equal to the result of
    converting the preceding characters in the sequence.

and change the text in Table 54, row 4, the error row, under the heading Meaning, from

    state has invalid value

to

    an unspecified error has occurred