Possible 5.5 regression - error in template

[=anm=]

I have a custom column, with template shown below.
It's working fine in 5.4.2, but in 5.5 (and 5.6) there is suddenly a lot of errors instead of correct values.
As there were no changes on my part, nor were any of relevance mentioned in changelog, I presume a regression 5.4.2 -> 5.5.

In 5.5, template value is correctly returned for "/Fanfic/Dead" or "/Fanfic/Complete", but where in 5.4.2 was "/Fanfic", in 5.5 is TEMPLATE ERROR - EXCEPTION: Interpreter: Internal error evaluating an expression.

Windows 7 64b, Calibre portable 5.6.

Template:

program:

# Reader path
path = '';
# Book type: 0 = general, 1 = educational, 2 = FF, 3 = HP
type = 0;

age_dead = 1096;

type = in_list(field('tags'), ',', 'Educational', 1, type);
type = in_list(field('tags'), ',', 'FanFiction', 2, type);
type = in_list(field('tags'), ',', 'HP', 3, type);

path = switch(type,
 0, '/',
 1, '/Educational',
 3, '/HP',
 2, strcat(
  '/Fanfic',
  first_non_empty(
   in_list(field('tags'), ',', 'Completed', '/Complete', ''),
   cmp(
    days_between(today(), field('#ff_updated')),
    age_dead,
    '', '', '/Dead'
   )
  )
 ),
'');

[Kovid Goyal (kovid)]

Changing the component for this bug.

 assignee cbhaley
 status triaged

[Charles Haley (cbhaley)]

In this case you can work around the problem by adding ' ' (quote, space, quote) as the last argument to first_non_empty

[=anm= (anm-nov)]

Verified, no problem in 5.7.1.