Uplink with missing payload

I am using LMIC node from Lopes Parentes (@lnlp) on TTN network. My nodes are sending a 9 bytes payload every 15mn and everything works find except that once every 4 to 5 uplinks, the uplink doesn’t contain the payload that the device is sending. I noticed that is those cases, the frame port is not specified in the uplink, which means it was set to 0 and TTN dropped it, assuming it was a MAC command. Knowing that these uplinks with no payload are appearing on TTN console at the exact time the node should be sending an uplink, it looks like the uplink is coming from the node itself, not from TTN, and that the code is setting the frame port to 0 instead of anything different to 0. I couldn’t find information about where in the code the frame port is specified. Can somebody help ? Thanks.