[Exim] Unknown lookup type ldapauth?

What am I doing wrong? I have Exim 4.12 (this happened with Exim 4.14
too, so I downgraded but it didn't help).

login:
driver = plaintext
public_name = LOGIN
server_prompts = "Username:: : Password::"
server_condition = ${if eq {$2}{}{no}{${lookup ldapauth
{user="uid=${quote_ldap:$1},ou=People,dc=ysu,dc=edu" pass="$2"
ldap://ldap.ysu.edu/}{$value} fail}}}
server_set_id = uid=$1,ou=People,dc=ysu,dc=edu

(The server_condition is one line.)

When I try to authenticate (exim -bh) I get the prompts and then

expansion failed: unknown lookup type "ldapauth"

Any idea why?
Thanks,
John Dalbec