Can't Turn On Slow Query Logging

long_query_time = 1
log-slow-queries = /var/lib/mysql/slow_queries.log
Is the above syntax not correct for enabling slow query logging? All examples I've seen have the dashes in the second variable and underscores in the first.

When I restart MySQL with those lines in my.cnf, it fails to start, but writes nothing to its error log.
/var/lib/mysql/slow_queries.log exists, is owned by mysql, and has read/write permission.