diff --git a/frames/ack_frame.go b/frames/ack_frame.go
index a1fda41e2..bb9ba7fe2 100644
--- a/frames/ack_frame.go
+++ b/frames/ack_frame.go
@@ -64,7 +64,6 @@ func ParseAckFrame(r *bytes.Reader) (*AckFrame, error) {
 	if missingSequenceNumberDeltaLen == 0 {
 		missingSequenceNumberDeltaLen = 1
 	}
-	_ = missingSequenceNumberDeltaLen
 
 	frame.Entropy, err = r.ReadByte()
 	if err != nil {
@@ -120,28 +119,28 @@ func ParseAckFrame(r *bytes.Reader) (*AckFrame, error) {
 		}
 
 		for i := uint8(0); i < numRanges; i++ {
-			missingPacketSequenceNumberDeltaByte, err := r.ReadByte()
+			missingPacketSequenceNumberDelta, err := utils.ReadUintN(r, missingSequenceNumberDeltaLen)
 			if err != nil {
 				return nil, err
 			}
-			missingPacketSequenceNumberDelta := uint64(missingPacketSequenceNumberDeltaByte)
 
-			rangeLength, err := utils.ReadUintN(r, largestObservedLen)
+			rangeLengthByte, err := r.ReadByte()
 			if err != nil {
 				return nil, err
 			}
+			rangeLength := uint8(rangeLengthByte)
 
 			nackRange := ackhandler.NackRange{
 				Length: uint8(rangeLength + 1),
 			}
 			if i == 0 {
-				nackRange.FirstPacketNumber = frame.LargestObserved - protocol.PacketNumber(missingPacketSequenceNumberDelta+rangeLength)
+				nackRange.FirstPacketNumber = frame.LargestObserved - protocol.PacketNumber(missingPacketSequenceNumberDelta+uint64(rangeLength))
 			} else {
 				if missingPacketSequenceNumberDelta == 0 {
 					return nil, errors.New("ACK frame: Continues NACK ranges not yet implemented.")
 				}
 				lastNackRange := frame.NackRanges[len(frame.NackRanges)-1]
-				nackRange.FirstPacketNumber = lastNackRange.FirstPacketNumber - protocol.PacketNumber(missingPacketSequenceNumberDelta+rangeLength) - 1
+				nackRange.FirstPacketNumber = lastNackRange.FirstPacketNumber - protocol.PacketNumber(missingPacketSequenceNumberDelta+uint64(rangeLength)) - 1
 			}
 			frame.NackRanges = append(frame.NackRanges, &nackRange)
 		}
diff --git a/frames/ack_frame_test.go b/frames/ack_frame_test.go
index 67fb15204..6da7f2a6d 100644
--- a/frames/ack_frame_test.go
+++ b/frames/ack_frame_test.go
@@ -46,6 +46,18 @@ var _ = Describe("AckFrame", func() {
 			Expect(frame.NackRanges[0].Length).To(Equal(uint8(2)))
 		})
 
+		It("parses a frame containing one NACK range with a 48 bit missingPacketSequenceNumberDelta", func() {
+			b := bytes.NewReader([]byte{(0x4C | 0x20 | 0x03), 0x08, 0x37, 0x13, 0xAD, 0xFB, 0xCA, 0xDE, 0x72, 0x1, 0x1, 0x0, 0xc0, 0x15, 0x0, 0x0, 0x1, 0xFE, 0xCA, 0xEF, 0xBE, 0xAD, 0xDE, 0x3})
+			frame, err := ParseAckFrame(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(frame.LargestObserved).To(Equal(protocol.PacketNumber(0xDECAFBAD1337)))
+			Expect(frame.HasNACK()).To(Equal(true))
+			Expect(len(frame.NackRanges)).To(Equal(1))
+			// ToDo: check NACK range
+			Expect(frame.NackRanges[0].Length).To(Equal(uint8(4)))
+			Expect(b.Len()).To(Equal(0))
+		})
+
 		It("parses a frame containing multiple NACK ranges", func() {
 			b := bytes.NewReader([]byte{0x60, 0x2, 0xf, 0xb8, 0x1, 0x1, 0x0, 0xe5, 0x58, 0x4, 0x0, 0x3, 0x1, 0x6, 0x1, 0x2, 0x1, 0x0})
 			frame, err := ParseAckFrame(b)