Lua Script - openConnection (no replies)

Hi,
I am new to Workbench scripting and having trouble writing a basic Lua script to open a connection. Below is the script which yields this error:
luart: error: stdin:9: bad argument #3 to 'callFunction' (list expected, got no value)

Error executing script.



-- MySQL Workbench lua script
-- <description>
-- Written in MySQL Workbench 6.1.4


rdbmsConn = grtV.newObj("db.mgmt.Connection",
{name="rdbmsConnection", owner=app}) --owner=app

conn_id = grtM.callFunction ("DbMySQLQuery", "openConnection", rdbmsConn)
print(conn_id)
grtM.callFunction ("DbMySQLQuery", "closeConnection", conn_id)