diff --git a/dist/crc16.js b/dist/crc16.js
index d1514b2..017627d 100644
--- a/dist/crc16.js
+++ b/dist/crc16.js
@@ -45,6 +45,7 @@
     /**
      * Copied from Nordic's command line tool nrf.exe.
      * https://github.com/NordicSemiconductor/pc-nrfutil/blob/master/nordicsemi/dfu/crc16.py
+     * CRC-CCITT (0xFFFF).
      */
     return function (binaryData) {
         var crc = 0xFFFF;
diff --git a/dist/dfu.js b/dist/dfu.js
index 45f827d..28ce01f 100644
--- a/dist/dfu.js
+++ b/dist/dfu.js
@@ -166,7 +166,7 @@
         return new Promise(function(resolve, reject) {
             var versionChar = null;
             imageType = imageType || ImageType.Application;
-            initPacket.crc = crc;
+            initPacket.crc = crc || 0xFFFF; // Not used in mbed/ older bootloader revisions.
 
             connect(device)
             .then(function(chars) {