Unable to do "su -m" in OpenBSD 6.4

Today I upgraded my desktop PC from OpenBSD amd64 6.3 to 6.4.

Up to 6.3 I was able to simply execute "su -m" to become superuser while
keeping all my enviroment. Now under 6.4 I'm no longer able to do it:

casa:/home/giannici> su -m
Password:
su: permission denied (shell).


In the man page of su I see "As a security precaution, if the target
user's shell is a non-standard shell (as defined by getusershell(3)) and
the caller's real UID is non-zero, su will fail.".

But here is the output of "/etc/shells":

casa:/home/giannici> cat /etc/shells
# $OpenBSD: shells,v 1.8 2009/02/14 17:06:40 sobrado Exp $
#
# list of acceptable shells for chpass(1).
# ftpd(8) will not allow users to connect who are not using
# one of these shells, unless the user is listed in /etc/ftpchroot.
/bin/sh
/bin/csh
/bin/ksh
/usr/local/bin/bash
/usr/local/bin/tcsh

And here is the first line of vipw:

root:$2b$XXXXXXXXXXXXXXXXSomeothercharacters:0:0:daemon:0:0:Charlie
&:/root:/bin/ksh


Why I can do "su" but I cannot do "su -m" anymore?

Thanks.