diff --git a/scapy/contrib/cdp.py b/scapy/contrib/cdp.py
index 51e768c59a0d1fe390bcc1aeaee70cef7827ff52..fc4dad4ea6e3ce4ae49d3accb1555494490fbba7 100644
--- a/scapy/contrib/cdp.py
+++ b/scapy/contrib/cdp.py
@@ -303,9 +303,8 @@ class _CDPChecksum:
 
     def post_build(self, pkt, pay):
         p = pkt + pay
-        ck_p = self._check_len(p)
         if self.cksum is None:
-            cksum = checksum(ck_p)
+            cksum = checksum(self._check_len(p))
             p = p[:2] + struct.pack("!H", cksum) + p[4:]
         return p