diff --git a/internal/ackhandler/received_packet_handler.go b/internal/ackhandler/received_packet_handler.go
index 8af213245..93931b4eb 100644
--- a/internal/ackhandler/received_packet_handler.go
+++ b/internal/ackhandler/received_packet_handler.go
@@ -74,7 +74,7 @@ func (h *receivedPacketHandler) ReceivedPacket(packetNumber protocol.PacketNumbe
 	}
 
 	isMissing := h.isMissing(packetNumber)
-	if packetNumber > h.largestObserved {
+	if packetNumber >= h.largestObserved {
 		h.largestObserved = packetNumber
 		h.largestObservedReceivedTime = rcvTime
 	}
diff --git a/internal/ackhandler/received_packet_handler_test.go b/internal/ackhandler/received_packet_handler_test.go
index d845efe86..7e7ddd06e 100644
--- a/internal/ackhandler/received_packet_handler_test.go
+++ b/internal/ackhandler/received_packet_handler_test.go
@@ -95,17 +95,17 @@ var _ = Describe("receivedPacketHandler", func() {
 			}
 
 			It("always queues an ACK for the first packet", func() {
-				err := handler.ReceivedPacket(1, time.Time{}, false)
-				Expect(err).ToNot(HaveOccurred())
+				Expect(handler.ReceivedPacket(1, time.Now(), false)).To(Succeed())
 				Expect(handler.ackQueued).To(BeTrue())
 				Expect(handler.GetAlarmTimeout()).To(BeZero())
+				Expect(handler.GetAckFrame().DelayTime).To(BeNumerically("~", 0, time.Second))
 			})
 
 			It("works with packet number 0", func() {
-				err := handler.ReceivedPacket(0, time.Time{}, false)
-				Expect(err).ToNot(HaveOccurred())
+				Expect(handler.ReceivedPacket(0, time.Now(), false)).To(Succeed())
 				Expect(handler.ackQueued).To(BeTrue())
 				Expect(handler.GetAlarmTimeout()).To(BeZero())
+				Expect(handler.GetAckFrame().DelayTime).To(BeNumerically("~", 0, time.Second))
 			})
 
 			It("queues an ACK for every second retransmittable packet at the beginning", func() {