microcoap on ARDUINO DUE: problem in visualization of resources on client

Hi at all, I am developing a microcoap server on ARDUINO DUE, that contains several resources (endpoints). For client, I am using Copper, Mozilla Firefox extension. When I do "DISCOVER" on client (to visualize the scheme of resources), the server receive the request and process the request, but on the client appears this message (with "OK" button):

ERROR: UdpClient.onDataAvailable [TypeError: b[k] is undefined]

(Make sure to use the correct CoAP version.)

The resources aren' t visualized. After few second, the client try again to send the same request, then the server process the request, but on the client appears the same ERROR message. The client try again to send the same request for a maximum of 4 times. When I visualize the packet' s payload of response of the server (in coap_make_response() function), I think that it' s contains the right things. The ERROR message window makes me think that the response of the server arrives to the client. Can you help me to resolve this problem? Is the buffer of ARDUINO too small or is the client that can' t decode the packet' s payload received? Or is it another problem?