diff --git a/ethermess.c b/ethermess.c
index 954955c..ec99b3a 100644
--- a/ethermess.c
+++ b/ethermess.c
@@ -73,6 +73,12 @@ ssize_t msgid_cache_lookup(const unsigned char mac[6]) {
 ssize_t msgid_cache_add(const unsigned char mac[6]) {
 	ssize_t index = next_slot++;
 
+	// If we are adding into a new slot (instead of overwriting one),
+	// expand the fill pointer
+	if (msgid_cache_fill < index) {
+		msgid_cache_fill++;
+	}
+
 	memcpy(msgid_cache[index].other_mac, mac, sizeof(msgid_cache[index].other_mac));
 	msgid_cache[index].know_send = false;
 	msgid_cache[index].know_receive = false;