diff --git a/default.nix b/default.nix
index d5acbbdd7..044f59e7b 100644
--- a/default.nix
+++ b/default.nix
@@ -80,6 +80,10 @@ mach-nix.buildPythonPackage {
         sha256 = "1md9i2fx1ya7mgcj9j01z58hs3q9pj4ch5is5b5kq4v86cf6x33x";
       })
     ];
-  };
 
+    # Remove a click-default-group patch for a test suite problem which no
+    # longer applies because the project apparently no longer has a test suite
+    # in its source distribution.
+    click-default-group.patches = [];
+  };
 }
diff --git a/tests.nix b/tests.nix
new file mode 100644
index 000000000..364407e87
--- /dev/null
+++ b/tests.nix
@@ -0,0 +1,26 @@
+let
+  sources = import nix/sources.nix;
+in
+# See default.nix for documentation about parameters.
+{ pkgsVersion ? "nixpkgs-21.11"
+, pkgs ? import sources.${pkgsVersion} { }
+, pypiData ? sources.pypi-deps-db
+, pythonVersion ? "python37"
+, mach-nix ? import sources.mach-nix {
+    inherit pkgs pypiData;
+    python = pythonVersion;
+  }
+}@args:
+let
+  # Get the package with all of its test requirements.
+  tahoe-lafs = import ./. (args // { extras = [ "test" ]; });
+
+  # Put it into a Python environment.
+  python-env = pkgs.${pythonVersion}.withPackages (ps: [
+    tahoe-lafs
+  ]);
+in
+# Make a derivation that runs the unit test suite.
+pkgs.runCommand "tahoe-lafs-tests" { } ''
+  ${python-env}/bin/python -m twisted.trial -j $NIX_BUILD_CORES allmydata
+''