v9.1, DROP TRIGGER IF EXISTS behaving oddly

I'm attempting to drop a trigger that may or may not exist, so am using
the "IF EXISTS" clause.  This works fine for tables, views, functions,
domains, and types, but for some reason seems to be ignored for
triggers.  I'd expect to see more about this online if it were a bug,
so I'm thinking I may be missing something obvious.

Example:
DROP TRIGGER IF EXISTS udf_customer_update_trigger ON customer;

Expected Output:
NOTICE:  trigger "udf_customer_update_trigger" does not exist, skipping

Observed Output:
ERROR:  relation "udf_customer_update_trigger" does not exist

Environment:
CentOS 6.6
postgresql91-server-9.1.14-1PGDG.rhel6.x86_64

Thanks,
Michael