diff --git a/scapy/layers/inet6.py b/scapy/layers/inet6.py
index 6f9f16b..d61c621 100644
--- a/scapy/layers/inet6.py
+++ b/scapy/layers/inet6.py
@@ -1089,9 +1089,9 @@ class IPv6ExtHdrSegmentRouting(_IPv6ExtHdr):
                     BitField("unused2", 0, 3),
                     ShortField("tag", 0),
                     IP6ListField("addresses", ["::1"],
-                        count_from=lambda pkt: pkt.lastentry),
+                        count_from=lambda pkt: pkt.lastentry+1),
                     PacketListField("tlv_objects", [], IPv6ExtHdrSegmentRoutingTLV,
-                        length_from=lambda pkt: 8*pkt.len - 16*pkt.lastentry) ]
+                        length_from=lambda pkt: 8*pkt.len - 16*(pkt.lastentry+1)) ]
 
     overload_fields = { IPv6: { "nh": 43 } }
 
@@ -1119,7 +1119,10 @@ class IPv6ExtHdrSegmentRouting(_IPv6ExtHdr):
             pkt = pkt[:3] + struct.pack("B", tmp_len) + pkt[4:]
 
         if self.lastentry is None:
-            pkt = pkt[:4] + struct.pack("B", len(self.addresses)) + pkt[5:]
+            tmp_len = len(self.addresses)
+            if tmp_len:
+                tmp_len -= 1
+            pkt = pkt[:4] + struct.pack("B", tmp_len) + pkt[5:]
 
         return _IPv6ExtHdr.post_build(self, pkt, pay)