diff --git a/ackhandler/outgoing_packet_ack_handler.go b/ackhandler/outgoing_packet_ack_handler.go
index 67ab085fa..9b47ddc3b 100644
--- a/ackhandler/outgoing_packet_ack_handler.go
+++ b/ackhandler/outgoing_packet_ack_handler.go
@@ -35,8 +35,7 @@ func (h *outgoingPacketAckHandler) SentPacket(packet *Packet) error {
 	}
 
 	var lastPacketEntropy EntropyAccumulator
-	// ToDo: write tests for this
-	if packet.PacketNumber == 0 {
+	if packet.PacketNumber == 1 {
 		lastPacketEntropy = EntropyAccumulator(0)
 	} else {
 		if h.highestInOrderAckedPacketNumber == packet.PacketNumber-1 {
diff --git a/ackhandler/outgoing_packet_ack_handler_test.go b/ackhandler/outgoing_packet_ack_handler_test.go
index 680e13c16..2f6b8948a 100644
--- a/ackhandler/outgoing_packet_ack_handler_test.go
+++ b/ackhandler/outgoing_packet_ack_handler_test.go
@@ -56,6 +56,26 @@ var _ = Describe("AckHandler", func() {
 			Expect(handler.packetHistory).ToNot(HaveKey(protocol.PacketNumber(2)))
 			Expect(handler.packetHistory).ToNot(HaveKey(protocol.PacketNumber(2)))
 		})
+
+		It("correctly calculates the entropy, even if the last packet has already been ACKed", func() {
+			packet1 := Packet{PacketNumber: 1, Plaintext: []byte{0x13, 0x37}, EntropyBit: true}
+			packet2 := Packet{PacketNumber: 2, Plaintext: []byte{0xBE, 0xEF}, EntropyBit: true}
+			err := handler.SentPacket(&packet1)
+			Expect(err).ToNot(HaveOccurred())
+			expectedEntropy := EntropyAccumulator(0)
+			expectedEntropy.Add(packet1.PacketNumber, packet1.EntropyBit)
+			ack := frames.AckFrame{
+				LargestObserved: 1,
+				Entropy:         byte(expectedEntropy),
+			}
+			err = handler.ReceivedAck(&ack)
+			Expect(err).ToNot(HaveOccurred())
+			err = handler.SentPacket(&packet2)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(handler.lastSentPacketNumber).To(Equal(protocol.PacketNumber(2)))
+			expectedEntropy.Add(packet2.PacketNumber, packet2.EntropyBit)
+			Expect(handler.packetHistory[2].Entropy).To(Equal(expectedEntropy))
+		})
 	})
 
 	Context("ACK handling", func() {