diff --git a/source/nwm_misc/ikcp.c b/source/nwm_misc/ikcp.c
index 363238c..432d70a 100644
--- a/source/nwm_misc/ikcp.c
+++ b/source/nwm_misc/ikcp.c
@@ -605,6 +605,10 @@ int ikcp_input(ikcpcb *kcp, char *data, int size)
 		IUINT16 nack_start = (val >> count_nbits) & ((1 << PID_NBITS) - 1);
 		IUINT16 nack_count_0 = size == 1 ? (1 << (PID_NBITS - 2)) - 1 : val & ((1 << count_nbits) - 1);
 
+		if (kcp->n_nacks >= sizeof(kcp->nacks) / sizeof(*kcp->nacks)) {
+			return -4;
+		}
+
 		kcp->nacks[kcp->n_nacks][0] = nack_start;
 		kcp->nacks[kcp->n_nacks][1] = nack_count_0;
 		++kcp->n_nacks;
diff --git a/source/nwm_misc/ikcp.h b/source/nwm_misc/ikcp.h
index 829bbf0..2405b14 100644
--- a/source/nwm_misc/ikcp.h
+++ b/source/nwm_misc/ikcp.h
@@ -358,8 +358,8 @@ struct IKCPCB
 #ifdef CHECK_PID
 	struct BitSet4096Mem pid_bs;
 #endif
-	// Worst case every other packet is nack
-	u16 nacks[DIV_ROUND_UP(ARQ_PREFERRED_COUNT_MAX, 2)][2];
+	// Worst case between every other packet is nack
+	u16 nacks[ARQ_PREFERRED_COUNT_MAX][2];
 	u16 n_nacks;
 };