diff --git a/src/avalanche/processor.cpp b/src/avalanche/processor.cpp
--- a/src/avalanche/processor.cpp
+++ b/src/avalanche/processor.cpp
@@ -252,7 +252,10 @@
         // Generate the delegation to the session key.
         DelegationBuilder dgb(*peerData->proof);
         if (sessionKey.GetPubKey() != peerData->proof->getMaster()) {
-            dgb.addLevel(masterKey, sessionKey.GetPubKey());
+            if (!dgb.addLevel(masterKey, sessionKey.GetPubKey())) {
+                error = _("The private key does not match the proof.");
+                return nullptr;
+            }
         }
         peerData->delegation = dgb.build();
     }
diff --git a/test/functional/abc_rpc_avalancheproof.py b/test/functional/abc_rpc_avalancheproof.py
--- a/test/functional/abc_rpc_avalancheproof.py
+++ b/test/functional/abc_rpc_avalancheproof.py
@@ -366,6 +366,18 @@
                 match=ErrorMatch.PARTIAL_REGEX,
             )
 
+        # Master private key mismatch
+        random_privkey = ECKey()
+        random_privkey.generate()
+        node.assert_start_raises_init_error(
+            self.extra_args[0] + [
+                "-avaproof={}".format(proof),
+                "-avamasterkey={}".format(
+                    bytes_to_wif(random_privkey.get_bytes())),
+            ],
+            expected_msg="Error: The private key does not match the proof.",
+        )
+
 
 if __name__ == '__main__':
     AvalancheProofTest().main()