diff --git a/bors.toml b/bors.toml
index b750afa..c063f8e 100644
--- a/bors.toml
+++ b/bors.toml
@@ -1,2 +1,4 @@
 cut_body_after = "" # don't include text from the PR body in the merge commit message
-status = [ "tests" ]
+status = [
+  "tests (ubuntu-latest)"
+]
diff --git a/direnvrc b/direnvrc
index d3dd839..b2e7d24 100644
--- a/direnvrc
+++ b/direnvrc
@@ -252,27 +252,29 @@ use_flake() {
 use_nix() {
   _nix_direnv_preflight
 
-  local layout_dir path
-  path=$(_nix_direnv_realpath "$("${NIX_BIN_PREFIX}nix-instantiate" --find-file nixpkgs)")
+  local layout_dir path version
   layout_dir=$(direnv_layout_dir)
+  path=$("${NIX_BIN_PREFIX}nix-instantiate" --find-file nixpkgs 2>/dev/null)
+  if [[ -n "$path" ]]; then
+    path=$(_nix_direnv_realpath "$path")
 
-  local version
-  if [[ -f "${path}/.version-suffix" ]]; then
-    version=$(< "${path}/.version-suffix")
-  elif [[ -f "${path}/.git/HEAD" ]]; then
-    local head
-    read -r head < "${path}/.git/HEAD"
-    local regex="ref: (.*)"
-    if [[ "$head" =~  $regex ]]; then
-      read -r version < "${path}/.git/${BASH_REMATCH[1]}"
-    else
-      version="$head"
+    if [[ -f "${path}/.version-suffix" ]]; then
+      version=$(< "${path}/.version-suffix")
+    elif [[ -f "${path}/.git/HEAD" ]]; then
+      local head
+      read -r head < "${path}/.git/HEAD"
+      local regex="ref: (.*)"
+      if [[ "$head" =~  $regex ]]; then
+        read -r version < "${path}/.git/${BASH_REMATCH[1]}"
+      else
+        version="$head"
+      fi
+    elif [[ -f "${path}/.version" && "${path}" == "/nix/store/"* ]]; then
+      # borrow some bits from the store path
+      local version_prefix
+      read -r version_prefix < "${path}/.version"
+      version="${version_prefix}-${path:11:16}"
     fi
-  elif [[ -f "${path}/.version" && "${path}" == "/nix/store/"* ]]; then
-    # borrow some bits from the store path
-    local version_prefix
-    read -r version_prefix < "${path}/.version"
-    version="${version_prefix}-${path:11:16}"
   fi
 
   local profile
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..b470684
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,20 @@
+[tool.ruff]
+line-length = 88
+
+select = ["E", "F", "I"]
+ignore = [ "E501" ]
+
+[tool.mypy]
+python_version = "3.10"
+warn_redundant_casts = true
+disallow_untyped_calls = true
+disallow_untyped_defs = true
+no_implicit_optional = true
+
+[[tool.mypy.overrides]]
+module = "setuptools.*"
+ignore_missing_imports = true
+
+[[tool.mypy.overrides]]
+module = "pytest.*"
+ignore_missing_imports = true
diff --git a/run-tests.nix b/run-tests.nix
index 189198e..ae908cc 100644
--- a/run-tests.nix
+++ b/run-tests.nix
@@ -4,6 +4,8 @@
   direnv,
   mypy,
   python3,
+  lib,
+  ruff
 }:
 writeScript "run-tests" ''
   set -e
@@ -11,12 +13,12 @@ writeScript "run-tests" ''
   echo run shellcheck
   ${shellcheck}/bin/shellcheck direnvrc
   echo run black
-  LC_ALL=en_US.utf-8 ${python3.pkgs.black}/bin/black --check .
-  echo run flake8
-  ${python3.pkgs.flake8}/bin/flake8 --ignore E501 tests
+  LC_ALL=en_US.utf-8 ${lib.getExe python3.pkgs.black} --check .
+  echo run ruff
+  ${lib.getExe ruff} tests
   echo run mypy
-  ${mypy}/bin/mypy tests
+  ${lib.getExe mypy} tests
 
   echo run unittest
-  ${python3.pkgs.pytest}/bin/pytest .
+  ${lib.getExe python3.pkgs.pytest} .
 ''
diff --git a/tests/direnv_project.py b/tests/direnv_project.py
index 17c4c50..784eaaa 100644
--- a/tests/direnv_project.py
+++ b/tests/direnv_project.py
@@ -8,7 +8,6 @@ from tempfile import TemporaryDirectory
 from typing import Iterator
 
 import pytest
-
 from procs import run
 
 
@@ -40,6 +39,8 @@ def direnv_project(test_root: Path, project_root: Path) -> Iterator[DirenvProjec
     with TemporaryDirectory() as _dir:
         dir = Path(_dir) / "proj"
         shutil.copytree(test_root / "testenv", dir)
+        shutil.copyfile(project_root / "flake.nix", dir / "flake.nix")
+        shutil.copyfile(project_root / "flake.lock", dir / "flake.lock")
         nix_direnv = project_root / "direnvrc"
 
         c = DirenvProject(Path(dir), nix_direnv)
diff --git a/tests/procs.py b/tests/procs.py
index 81cf150..7f8cf5e 100644
--- a/tests/procs.py
+++ b/tests/procs.py
@@ -1,9 +1,8 @@
 #!/usr/bin/env python3
 
 import subprocess
-from typing import List, Union, IO, Any
 from pathlib import Path
-
+from typing import IO, Any, List, Optional, Union
 
 _FILE = Union[None, int, IO[Any]]
 _DIR = Union[None, Path, str]
@@ -16,10 +15,11 @@ def run(
     cwd: _DIR = None,
     stderr: _FILE = None,
     stdout: _FILE = None,
+    env: Optional[dict[str, str]] = None,
 ) -> subprocess.CompletedProcess:
     if cwd is not None:
         print(f"cd {cwd}")
     print("$ " + " ".join(cmd))
     return subprocess.run(
-        cmd, text=text, check=check, cwd=cwd, stderr=stderr, stdout=stdout
+        cmd, text=text, check=check, cwd=cwd, stderr=stderr, stdout=stdout, env=env
     )
diff --git a/tests/test_gc.py b/tests/test_gc.py
index fefda77..2982128 100644
--- a/tests/test_gc.py
+++ b/tests/test_gc.py
@@ -1,11 +1,11 @@
 #!/usr/bin/env python2
 
-import sys
 import subprocess
+import sys
 import unittest
 
-from procs import run
 from direnv_project import DirenvProject
+from procs import run
 
 
 def common_test(direnv_project: DirenvProject) -> None:
diff --git a/tests/test_use_nix.py b/tests/test_use_nix.py
index c9495f8..8144a62 100644
--- a/tests/test_use_nix.py
+++ b/tests/test_use_nix.py
@@ -1,20 +1,27 @@
 #!/usr/bin/env python2
 
-import sys
+import os
 import subprocess
+import sys
 import unittest
+from typing import Optional
 
-from procs import run
 from direnv_project import DirenvProject
+from procs import run
 
 
-def direnv_exec(direnv_project: DirenvProject, cmd: str) -> None:
+def direnv_exec(
+    direnv_project: DirenvProject, cmd: str, env: Optional[dict[str, str]] = None
+) -> None:
+    args = ["direnv", "exec", str(direnv_project.dir), "sh", "-c", cmd]
+    print("$ " + " ".join(args))
     out = run(
-        ["direnv", "exec", str(direnv_project.dir), "sh", "-c", cmd],
+        args,
         stderr=subprocess.PIPE,
         stdout=subprocess.PIPE,
         check=False,
         cwd=direnv_project.dir,
+        env=env,
     )
     sys.stdout.write(out.stdout)
     sys.stderr.write(out.stderr)
@@ -28,6 +35,13 @@ def test_attrs(direnv_project: DirenvProject) -> None:
     direnv_exec(direnv_project, "echo $THIS_IS_A_SUBSHELL")
 
 
+def test_no_nix_path(direnv_project: DirenvProject) -> None:
+    direnv_project.setup_envrc("use nix --argstr someArg OK")
+    env = os.environ.copy()
+    del env["NIX_PATH"]
+    direnv_exec(direnv_project, "echo $SHOULD_BE_SET", env=env)
+
+
 def test_args(direnv_project: DirenvProject) -> None:
     direnv_project.setup_envrc("use nix --argstr someArg OK")
     direnv_exec(direnv_project, "echo $SHOULD_BE_SET")
diff --git a/tests/testenv/shell.nix b/tests/testenv/shell.nix
index 05d0a7a..a0456f6 100644
--- a/tests/testenv/shell.nix
+++ b/tests/testenv/shell.nix
@@ -1,6 +1,9 @@
-{ pkgs ? import <nixpkgs> { }, someArg ? null, shellHook ? ''
+{ pkgs ? import (builtins.getFlake (toString ./.)).inputs.nixpkgs { }
+, someArg ? null
+, shellHook ? ''
   echo "Executing shellHook."
-'' }:
+''
+}:
 pkgs.mkShellNoCC {
   inherit shellHook;