diff --git a/lib/with_advisory_lock/postgresql.rb b/lib/with_advisory_lock/postgresql.rb
index ae369cb..343470a 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.connection.transaction_open?
+      
       execute_successful?(pg_function)
     end
 
diff --git a/test/transaction_test.rb b/test/transaction_test.rb
index e0e224f..65fee52 100644
--- a/test/transaction_test.rb
+++ b/test/transaction_test.rb
@@ -69,5 +69,15 @@ def pg_lock_count
       end
       assert_equal(0, pg_lock_count)
     end
+
+    specify 'transaction level locks fail if not in transaction' do
+      exception = assert_raises do
+        Tag.with_advisory_lock 'test', transaction: true do
+          raise 'should not get here'
+        end
+      end
+
+      assert_match(/#{Regexp.escape('requires transaction')}/, exception.message)
+    end
   end
 end