diff --git a/test/functional/test_framework/schnorr.py b/test/functional/test_framework/schnorr.py
--- a/test/functional/test_framework/schnorr.py
+++ b/test/functional/test_framework/schnorr.py
@@ -152,7 +152,7 @@
         if k > 0 and k < SECP256K1_ORDER:
             break
         K = hmac.HMAC(K, V+b'\x00', 'sha256').digest()
-        V = HMAC_K(V)
+        V = hmac.HMAC(K, V, 'sha256').digest()
     return k