Modbus TCP Writes Fail after Reads

[admin]

"method called on invalid context" means that the connection is not open or already closed.
Both examples will produce this error:

Code:

mb = require('luamodbus').tcp()
mb:open(IP, PORT)
mb:connect()
mb:close()

res, err = mb:readregisters(0)

Code:

mb = require('luamodbus').tcp()
res, err = mb:readregisters(0)