diff --git a/scapy/layers/inet6.py b/scapy/layers/inet6.py
index a3bf89db69a..62dd93a2a9a 100644
--- a/scapy/layers/inet6.py
+++ b/scapy/layers/inet6.py
@@ -1139,7 +1139,9 @@ def defragment6(packets):
         offset = 8 * q.offset
         if offset != len(fragmentable):
             warning("Expected an offset of %d. Found %d. Padding with XXXX" % (len(fragmentable), offset))  # noqa: E501
-        frag_data_len = p[IPv6].plen - frag_hdr_len
+        frag_data_len = p[IPv6].plen
+        if frag_data_len is not None:
+            frag_data_len -= frag_hdr_len
         fragmentable += b"X" * (offset - len(fragmentable))
         fragmentable += raw(q.payload)[:frag_data_len]