diff --git a/include/ec_commands.h b/include/ec_commands.h
index 153ea56a84..c7bc3c8b8e 100644
--- a/include/ec_commands.h
+++ b/include/ec_commands.h
@@ -551,6 +551,12 @@ enum host_event_code {
 	/* Keyboard recovery combo with hardware reinitialization */
 	EC_HOST_EVENT_KEYBOARD_RECOVERY_HW_REINIT = 30,
 
+	/*
+	 * Reserve this last bit to indicate that at least one bit in a
+	 * secondary host event word is set.  See crbug.com/633646.
+	 */
+	EC_HOST_EVENT_EXTENDED = 31,
+
 	/*
 	 * The high bit of the event mask is not used as a host event code.  If
 	 * it reads back as set, then the entire event mask should be