diff --git a/opentimestamps/core/secp256k1.py b/opentimestamps/core/secp256k1.py
index d9d5194..58cfcc5 100644
--- a/opentimestamps/core/secp256k1.py
+++ b/opentimestamps/core/secp256k1.py
@@ -30,9 +30,10 @@ def _do_op_call(self, msg):
             raise MsgValueError("Missing secp256k1 point")
 
         pt = Point.decode(msg[0:33])
+        assert(pt.encode() == msg[0:33])
 
         hasher = hashlib.sha256()
-        hasher.update(pt.encode())
+        hasher.update(msg[0:33])
         hasher.update(msg[33:])
         tweak = int.from_bytes(hasher.digest(), 'big')
         tweak_pt = SECP256K1_GEN.scalar_mul(tweak)