diff --git a/nix/tahoe-lafs.nix b/nix/tahoe-lafs.nix
index d82c97db7..c26572ec4 100644
--- a/nix/tahoe-lafs.nix
+++ b/nix/tahoe-lafs.nix
@@ -4,7 +4,7 @@
 , setuptools, setuptoolsTrial, pyasn1, zope_interface
 , service-identity, pyyaml, magic-wormhole, treq, appdirs
 , beautifulsoup4, eliot, autobahn, cryptography
-, html5lib, future
+, html5lib, future, pyutil
 }:
 python.pkgs.buildPythonPackage rec {
   version = "1.14.0.dev";
@@ -50,7 +50,7 @@ python.pkgs.buildPythonPackage rec {
     setuptoolsTrial pyasn1 zope_interface
     service-identity pyyaml magic-wormhole treq
     eliot autobahn cryptography setuptools
-    future
+    future pyutil
   ];
 
   checkInputs = with python.pkgs; [
diff --git a/setup.py b/setup.py
index 919c84818..047d08030 100644
--- a/setup.py
+++ b/setup.py
@@ -124,6 +124,9 @@ install_requires = [
 
     # Support for Python 3 transition
     "future >= 0.18.2",
+
+    # Utility code:
+    "pyutil >= 3.3.0",
 ]
 
 setup_requires = [