diff --git a/Tools/AP_Bootloader/app_comms.h b/Tools/AP_Bootloader/app_comms.h
index 7d006620b4..619f7597c5 100644
--- a/Tools/AP_Bootloader/app_comms.h
+++ b/Tools/AP_Bootloader/app_comms.h
@@ -16,6 +16,11 @@ struct app_bootloader_comms {
     uint8_t path[201];
 };
 
+#ifndef FW_MAJOR
+#define FW_MAJOR 0
+#define FW_MINOR 0
+#endif
+
 /*
  the app_descriptor stored in flash in the main firmware and is used
  by the bootloader to confirm that the firmware is not corrupt and is
@@ -23,21 +28,21 @@ struct app_bootloader_comms {
  are filled in by set_app_descriptor() in the waf build
  */
 struct app_descriptor {
-    uint8_t sig[8];
+    uint8_t sig[8] = { 0x40, 0xa2, 0xe4, 0xf1, 0x64, 0x68, 0x91, 0x06 };
     // crc1 is the crc32 from firmware start to start of image_crc1
-    uint32_t image_crc1;
+    uint32_t image_crc1 = 0;
     // crc2 is the crc32 from the start of version_major to the end of the firmware
-    uint32_t image_crc2;
+    uint32_t image_crc2 = 0;
     // total size of firmware image in bytes
-    uint32_t image_size;
-    uint32_t git_hash;
+    uint32_t image_size = 0;
+    uint32_t git_hash = 0;
     // software version number
-    uint8_t  version_major;
-    uint8_t version_minor;
+    uint8_t  version_major = FW_MAJOR;
+    uint8_t version_minor = FW_MINOR;
     // APJ_BOARD_ID (hardware version). This is also used in CAN NodeInfo
     // with high byte in HardwareVersion.major and low byte in HardwareVersion.minor
-    uint16_t  board_id;
-    uint8_t reserved[8];
+    uint16_t  board_id = APJ_BOARD_ID;
+    uint8_t reserved[8] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };
 };
 
 #define APP_DESCRIPTOR_TOTAL_LENGTH 36