I am running Apache 2.2.17 on Suse Linux 11. I have installed the mod_authn_dbd module to allow authentication using a MySQL database. Simple digest authentication works absolutely fine using the following directive:

AuthDBDUserRealmQuery "SELECT loginPassword FROM login WHERE loginUser = %s and loginRealm = %s"

However, I would like to both generalize this statement and prepare it for performance and security reasons. Thus, I used the following directives instead:

DBDPrepareSQL "SELECT loginPassword FROM login WHERE loginUser = %s and loginRealm = %s" digestLogin
AuthDBDUserRealmQuery digestLogin

These directives generate the following errors:

[...] [error] (20014)Internal error: DBD: failed to prepare SQL statements:
[...] [error] (20014)Internal error: DBD: failed to initialise

Why does my actual SQL statement work when used directly, but not when I try to prepare it?

(Note I have tried using '?' and '%%' in place of '%', to no effect.)