The driver can incorrectly send OP_QUERY to mongos with a read preference like:

{$readPreference:{mode:'secondary'}}

mongos 2.4 returns "BadValue: cannot canonicalize query". Later mongoses accept this format. Regardless, according to Server Selection Spec, if the query filter is empty it must be included anyway:

{$query:{}, $readPreference:{mode:'secondary'}}

This format works with all supported versions of mongos.