diff --git a/fec.go b/fec.go
index de58b15..576e45f 100644
--- a/fec.go
+++ b/fec.go
@@ -13,6 +13,7 @@ const (
 	typeData           = 0xf1
 	typeParity         = 0xf2
 	fecExpire          = 60000
+	rxFECMulti         = 3 // FEC keeps rxFECMulti* (dataShard+parityShard) ordered packets in memory
 )
 
 // fecPacket is a decoded FEC packet
@@ -50,19 +51,16 @@ type fecDecoder struct {
 	autoTune autoTune
 }
 
-func newFECDecoder(rxlimit, dataShards, parityShards int) *fecDecoder {
+func newFECDecoder(dataShards, parityShards int) *fecDecoder {
 	if dataShards <= 0 || parityShards <= 0 {
 		return nil
 	}
-	if rxlimit < dataShards+parityShards {
-		return nil
-	}
 
 	dec := new(fecDecoder)
-	dec.rxlimit = rxlimit
 	dec.dataShards = dataShards
 	dec.parityShards = parityShards
 	dec.shardSize = dataShards + parityShards
+	dec.rxlimit = rxFECMulti * dec.shardSize
 	codec, err := reedsolomon.New(dataShards, parityShards)
 	if err != nil {
 		return nil
@@ -76,17 +74,17 @@ func newFECDecoder(rxlimit, dataShards, parityShards int) *fecDecoder {
 
 // decode a fec packet
 func (dec *fecDecoder) decode(in fecPacket) (recovered [][]byte) {
-	// autotune
+	// sample to auto FEC tuner
 	if in.flag() == typeData {
 		dec.autoTune.Sample(true, in.seqid())
 	} else {
 		dec.autoTune.Sample(false, in.seqid())
 	}
 
-	// check if FEC parameters has mismatched
+	// check if FEC parameters is out of sync
 	var shouldTune bool
-	if int(in.seqid())%dec.shardSize < dec.dataShards { // should be data
-		if in.flag() != typeData {
+	if int(in.seqid())%dec.shardSize < dec.dataShards {
+		if in.flag() != typeData { // expect typeData
 			shouldTune = true
 		}
 	} else {
@@ -100,10 +98,11 @@ func (dec *fecDecoder) decode(in fecPacket) (recovered [][]byte) {
 		autoPS := dec.autoTune.FindPeriod(false)
 
 		// edges found, we can tune parameters now
-		if autoDS > 0 && autoPS > 0 {
+		if autoDS > 0 && autoPS > 0 && autoDS < 256 && autoPS < 256 {
 			dec.dataShards = autoDS
 			dec.parityShards = autoPS
 			dec.shardSize = autoDS + autoPS
+			dec.rxlimit = rxFECMulti * dec.shardSize
 			codec, err := reedsolomon.New(autoDS, autoPS)
 			if err != nil {
 				return nil
diff --git a/sess.go b/sess.go
index 21b0dd3..a902783 100644
--- a/sess.go
+++ b/sess.go
@@ -35,9 +35,6 @@ const (
 	// maximum packet size
 	mtuLimit = 1500
 
-	// FEC keeps rxFECMulti* (dataShard+parityShard) ordered packets in memory
-	rxFECMulti = 3
-
 	// accept backlog
 	acceptBacklog = 128
 )
@@ -155,7 +152,7 @@ func newUDPSession(conv uint32, dataShards, parityShards int, l *Listener, conn
 	}
 
 	// FEC codec initialization
-	sess.fecDecoder = newFECDecoder(rxFECMulti*(dataShards+parityShards), dataShards, parityShards)
+	sess.fecDecoder = newFECDecoder(dataShards, parityShards)
 	if sess.block != nil {
 		sess.fecEncoder = newFECEncoder(dataShards, parityShards, cryptHeaderSize)
 	} else {
@@ -689,6 +686,10 @@ func (s *UDPSession) kcpInput(data []byte) {
 
 				// lock
 				s.mu.Lock()
+				// if fecDecoder is not initialized, create one with default parameter
+				if s.fecDecoder == nil {
+					s.fecDecoder = newFECDecoder(1, 1)
+				}
 				recovers := s.fecDecoder.decode(f)
 				if f.flag() == typeData {
 					if ret := s.kcp.Input(data[fecHeaderSizePlus2:], true, s.ackNoDelay); ret != 0 {
@@ -1021,7 +1022,7 @@ func serveConn(block BlockCrypt, dataShards, parityShards int, conn net.PacketCo
 	l.dataShards = dataShards
 	l.parityShards = parityShards
 	l.block = block
-	l.fecDecoder = newFECDecoder(rxFECMulti*(dataShards+parityShards), dataShards, parityShards)
+	l.fecDecoder = newFECDecoder(dataShards, parityShards)
 	l.chSocketReadError = make(chan struct{})
 
 	// calculate header size