diff --git a/kwant/tests/test_builder.py b/kwant/tests/test_builder.py
index 6b18541d31cb4c3e2c7bb055eaae2613937b716f..920bc2ffa803b8a05b273484de461ae7e8b2a52b 100644
--- a/kwant/tests/test_builder.py
+++ b/kwant/tests/test_builder.py
@@ -302,7 +302,7 @@ def test_finalization():
 
     def set_hops(dest, sites):
         while len(dest) < n_hops:
-            a, b = rng.sample(sites, 2)
+            a, b = rng.sample(list(sites), 2)
             if (a, b) not in dest and (b, a) not in dest:
                 dest[a, b] = random_hopping_integral(rng)