clean-buffer-list-kill-never-regexps is a variable defined in `midnight.el'.
Its value is
("^ \\*Minibuf-.*\\*$")

List of regexp saying which buffers will never be killed at midnight.
See also `clean-buffer-list-kill-never-buffer-names'.
Killing is done by `clean-buffer-list'.
Note that this does override `clean-buffer-list-kill-regexps' and
`clean-buffer-list-kill-buffer-names' so a buffer matching any of these
two lists will NOT be killed if it also matches anything in this list.

You can customize this variable.