diff --git a/src/rx/rx.h b/src/rx/rx.h
index 1f51b57b3a..b330565228 100644
--- a/src/rx/rx.h
+++ b/src/rx/rx.h
@@ -739,7 +739,7 @@ struct rx_ackPacket {
 #define	RX_ACK_TYPE_ACK		1   /* I have this packet, although I may discard it later */
 
 /* The packet size transmitted for an acknowledge is adjusted to reflect the actual size of the acks array.  This macro defines the size */
-#define rx_AckDataSize(nAcks) (3 + offsetof(struct rx_ackPacket, acks[nAcks]))
+#define rx_AckDataSize(nAcks) (3 + nAcks + offsetof(struct rx_ackPacket, acks[0]))
 
 #define	RX_CHALLENGE_TIMEOUT	2   /* Number of seconds before another authentication request packet is generated */
 #define RX_CHALLENGE_MAXTRIES	50  /* Max # of times we resend challenge */