Why is this Web MIDI API code not detecting MIDI events?

I am writing a Chrome app using the current stable build of Chrome (42.0.2311.152). I have this problem on both Linux and my Chromebook.

I have a USB MIDI keyboard that is recognized but no MIDI events are detected. I have checked with other programs and am confident that my device is sending MIDI events.

Here's my code:

function midihandler( event ) {
  console.log("Received MIDI event");
  var msg = "Recieved MIDI event ";
  for (i=0 ; i < event.data.length ; i++) {
    msg += "0x" + event.data[i].toString(16) + " ";
  } 
  console.log(msg);
}

function setupMIDI() {
  navigator.requestMIDIAccess().then(
    function (m) {
       console.log("MIDI initialized");
       m.inputs.forEach(
         function (entry) {
          console.log("detected MIDI input device " + entry.name);
          entry.onmidimessage = midihandler;
          for (var key in entry) {
            console.log("entry." + key + " = " + entry[key]);
          }
        }
      );
    },
    function (msg) { console.log("error initializing MIDI: " + msg); }
    );
}


window.onload = function() {
  console.log("window.onload called");
  setupMIDI();
};

      

        
        
        
      

    

I am getting console output:

window.onload called
MIDI initialized
detected MIDI input device MPKmini2 MIDI 1
entry.onmidimessage = function midihandler( event ) {
  console.log("Received MIDI event");
  var msg = "Recieved MIDI event ";
  for (i=0 ; i < event.data.length ; i++) {
    msg += "0x" + event.data[i].toString(16) + " ";
  }
  console.log(msg);
}
entry.ondisconnect = null
entry.version = USB-Audio / ALSA library version 1.0.25
entry.type = input
entry.name = MPKmini2 MIDI 1
entry.manufacturer = AKAI
entry.id = 24:0 MPKmini2 MIDI 1
entry.addEventListener = function addEventListener() { [native code] }
entry.removeEventListener = function removeEventListener() { [native code] }
entry.dispatchEvent = function dispatchEvent() { [native code] }
detected MIDI input device Midi Through Port-0
entry.onmidimessage = function midihandler( event ) {
  console.log("Received MIDI event");
  var msg = "Recieved MIDI event ";
  for (i=0 ; i < event.data.length ; i++) {
    msg += "0x" + event.data[i].toString(16) + " ";
  }
  console.log(msg);
}
entry.ondisconnect = null
entry.version = ALSA library version 1.0.25
entry.type = input
entry.name = Midi Through Port-0
entry.manufacturer = 
entry.id = 14:0 Midi Through Port-0
entry.addEventListener = function addEventListener() { [native code] }
entry.removeEventListener = function removeEventListener() { [native code] }
entry.dispatchEvent = function dispatchEvent() { [native code] }

      

        
        
        
      

    

That is, the devices are recognized and the onmidimessage handler appears.

However, no events are detected when I press keys on my MIDI keyboard.

What am I doing wrong?