Variable expansion on %w

I'm following the guide for changing the password schema. Everything works as expected (i'm using a static passwd file), but when the plain password has a % inside, dovecot is trying to expand that, triggering an error:

Apr 11 21:33:55 mail02 dovecot: pop3(x)<3962994><4soGPNcVXsoln9W6>: Error: Failed to expand plugin setting plain_pass = 'xxxxx%Yxxxxxx!%': Unknown variable '%Y'

How can I block the variable expansion inside %w ?