diff --git a/tests/functional2/testlib/fixtures/nix.py b/tests/functional2/testlib/fixtures/nix.py
index f794ecf9a..4a7fbef68 100644
--- a/tests/functional2/testlib/fixtures/nix.py
+++ b/tests/functional2/testlib/fixtures/nix.py
@@ -46,6 +46,7 @@ class NixSettings:
         config = dedent(f"""
             show-trace = true
             sandbox = true
+            substituters =
             extra-sandbox-paths = {" ".join(env.path.to_sandbox_paths())}
         """)
         # Note: newline at the end is required due to nix being nix;