If you do an "all-text" search for a word, that
produces no match, a mysql error will be provoked.

mysql doesn't like this query:

    SELECT _issue.id
    FROM _issue
    WHERE _issue.__retired__ <>1 AND _issue.id
    IN ()

which is produced in back_mysql.filter():
the indexer returns search_matches = {} and the backend
checks:

    if search_matches is not None:

which obviously doesn't work.

I thought that:

     if not search_matches:
         ...

would catch both None and {}, but this didn't work. See
patch for my clumsy solution

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the patch doesn't fix everything. I still get an error, when
I'm using a very short word (<3 ch). I'll look deeper into
it when I'm back from my vacation in 2 weeks ;)

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The full-text indexer doesn't match text less than 3 characters. 
 
I've altered the code such that we test at the top of filter(): 
 
        # we can't match anything if search_matches is empty 
        if search_matches == {}: 
            return [] 
 
then the test later in the code, which is "search_matches is not 
None", is untouched.