diff --git a/lib/with_advisory_lock/postgresql.rb b/lib/with_advisory_lock/postgresql.rb
index ae369cb..bcb9b31 100644
--- a/lib/with_advisory_lock/postgresql.rb
+++ b/lib/with_advisory_lock/postgresql.rb
@@ -5,6 +5,9 @@ class PostgreSQL < Base
     # See http://www.postgresql.org/docs/9.1/static/functions-admin.html#FUNCTIONS-ADVISORY-LOCKS
     def try_lock
       pg_function = "pg_try_advisory#{transaction ? '_xact' : ''}_lock#{shared ? '_shared' : ''}"
+      
+      raise "#{pg_function} requires transaction" if transaction && !ActiveRecord::Base.within_transaction?
+      
       execute_successful?(pg_function)
     end