Calling cal-html-cursor-month from a *Calendar* buffer using H-m
triggers an error in cal-html-insert-agenda-days (in
lisp/calendar/cal-html.el) if there are no holidays in the given
month. Since this is true for August, I've just triggered it!

The problem is the

    (and holiday-list diary-list "<BR>\n")

(line 399 in current trunk) inside an insert. (insert NIL) is not
ok. Changing the line to

    (if (and holiday-list diary-list) "<BR>\n" "")

Fixes the issue.

Thanks,

Rupert