diff --git a/adsbus/json.c b/adsbus/json.c
index e5280c6..7c2d1f7 100644
--- a/adsbus/json.c
+++ b/adsbus/json.c
@@ -90,8 +90,10 @@ static bool json_parse_header(json_t *in, struct packet *packet, struct json_par
 	}
 
 	if (mlat_timestamp_mhz > UINT16_MAX ||
-			mlat_timestamp_max < 0 ||
-			rssi_max > UINT32_MAX) {
+			mlat_timestamp_mhz <= 0 ||
+			mlat_timestamp_max <= 0 ||
+			rssi_max > UINT32_MAX ||
+			rssi_max <= 0) {
 		return false;
 	}
 
diff --git a/adsbus/packet.c b/adsbus/packet.c
index b35f75d..cd7df3b 100644
--- a/adsbus/packet.c
+++ b/adsbus/packet.c
@@ -13,6 +13,7 @@ char *packet_type_names[] = {
 };
 
 static uint64_t packet_mlat_timestamp_scale_mhz_in(uint64_t timestamp, uint32_t mhz) {
+	assert(mhz > 0);
 	return timestamp * (PACKET_MLAT_MHZ / mhz);
 }
 
@@ -27,10 +28,12 @@ static uint64_t packet_mlat_timestamp_scale_width_in(uint64_t timestamp, uint64_
 }
 
 static uint64_t packet_mlat_timestamp_scale_mhz_out(uint64_t timestamp, uint64_t mhz) {
+	assert(mhz > 0);
 	return timestamp / (PACKET_MLAT_MHZ / mhz);
 }
 
 static uint64_t packet_mlat_timestamp_scale_width_out(uint64_t timestamp, uint64_t max) {
+	assert(max > 0);
 	return timestamp % max;
 }
 
@@ -43,10 +46,12 @@ uint64_t packet_mlat_timestamp_scale_out(uint64_t timestamp, uint64_t max, uint1
 }
 
 uint32_t packet_rssi_scale_in(uint32_t value, uint32_t max) {
+	assert(max > 0);
 	return value * (PACKET_RSSI_MAX / max);
 }
 
 uint32_t packet_rssi_scale_out(uint32_t value, uint32_t max) {
+	assert(max > 0);
 	return value / (PACKET_RSSI_MAX / max);
 }
 
diff --git a/adsbus/proto.c b/adsbus/proto.c
index 0a00108..5592154 100644
--- a/adsbus/proto.c
+++ b/adsbus/proto.c
@@ -91,11 +91,16 @@ static bool proto_parse_header(AdsbHeader *header, struct packet *packet, struct
 		return false;
 	}
 
+	state->have_header = true;
 	fprintf(stderr, "R %s: Connected to server ID: %s\n", packet->source_id, header->server_id);
 	return true;
 }
 
 static bool proto_parse_packet(AdsbPacket *in, struct packet *packet, struct proto_parser_state *state, size_t len) {
+	if (!state->have_header) {
+		return false;
+	}
+
 	if (in->payload.len != len) {
 		return false;
 	}