diff --git a/src/interface/efi/efi_watchdog.c b/src/interface/efi/efi_watchdog.c
index 2876dc022..7061f81d8 100644
--- a/src/interface/efi/efi_watchdog.c
+++ b/src/interface/efi/efi_watchdog.c
@@ -44,7 +44,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define WATCHDOG_TIMEOUT_SECS ( 5 * 60 )
 
 /** Watchdog code (to be logged on watchdog timeout) */
-#define WATCHDOG_CODE 0x6950584544454144
+#define WATCHDOG_CODE 0x6950584544454144ULL
 
 /** Watchdog data (to be logged on watchdog timeout) */
 #define WATCHDOG_DATA L"iPXE";