diff --git a/Makefile b/Makefile
index f473784..a535518 100644
--- a/Makefile
+++ b/Makefile
@@ -12,7 +12,9 @@ DOCKER_DEB_TEST := sh -euxc ' \
 	&& tmp=$$(mktemp -d) \
 	&& cp -r /mnt/* "$$tmp" \
 	&& cd "$$tmp" \
-	&& pip install pytest \
+	&& pip install --upgrade pip \
+	&& /usr/local/bin/pip install --upgrade setuptools distribute \
+	&& /usr/local/bin/pip install -r requirements-dev.txt \
 	&& py.test tests/ \
 	&& exec dumb-init /mnt/tests/test-zombies \
 '
@@ -26,8 +28,10 @@ DOCKER_PYTHON_TEST := sh -uexc ' \
 	&& cd "$$tmp" \
 	&& python setup.py clean \
 	&& python setup.py sdist \
-	&& pip install -vv dist/*.tar.gz \
-	&& pip install pytest \
+	&& pip install --upgrade pip \
+	&& /usr/local/bin/pip install --upgrade setuptools distribute \
+	&& /usr/local/bin/pip install -vv dist/*.tar.gz \
+	&& /usr/local/bin/pip install -r requirements-dev.txt \
 	&& py.test tests/ \
 	&& exec dumb-init /mnt/tests/test-zombies \
 '
diff --git a/debian/control b/debian/control
index 55d74d4..17fa77e 100644
--- a/debian/control
+++ b/debian/control
@@ -3,7 +3,8 @@ Section: utils
 Priority: extra
 Maintainer: Chris Kuehl <ckuehl@yelp.com>
 Uploaders: Kent Wills <rkwills@yelp.com>
-Build-Depends: debhelper (>= 7), gcc, fakeroot, python, python-pytest
+Build-Depends: debhelper (>= 7), gcc, fakeroot, python, python-pytest,
+ python-mock
 Standards-Version: 3.9.6
 
 Package: dumb-init
diff --git a/requirements-dev.txt b/requirements-dev.txt
index 2dac807..6e596f7 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -1,3 +1,4 @@
+mock
 pre-commit>=0.5.0
 pytest
 pytest-timeout
diff --git a/tests/conftest.py b/tests/conftest.py
index 86b8c2a..2f10460 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,35 +1,48 @@
 import os
 
+import mock
 import pytest
 
 
-@pytest.fixture(autouse=True, scope='function')
+@pytest.yield_fixture(autouse=True, scope='function')
 def clean_environment():
-    """Ensure tests don't pollute each others' environment variables."""
-    os.environ.pop('DUMB_INIT_DEBUG', None)
-    os.environ.pop('DUMB_INIT_SETSID', None)
+    """Ensure all tests start with a clean environment.
+
+    Even if tests properly clean up after themselves, we still need this in
+    case the user runs tests with an already-polluted environment.
+    """
+    with mock.patch.dict(
+        os.environ,
+        {'DUMB_INIT_DEBUG': '', 'DUMB_INIT_SETSID': ''},
+    ):
+        yield
 
 
-@pytest.fixture(params=['1', '0'])
+@pytest.yield_fixture(params=['1', '0'])
 def both_debug_modes(request):
-    os.environ['DUMB_INIT_DEBUG'] = request.param
+    with mock.patch.dict(os.environ, {'DUMB_INIT_DEBUG': request.param}):
+        yield
 
 
-@pytest.fixture
+@pytest.yield_fixture
 def debug_disabled():
-    os.environ['DUMB_INIT_DEBUG'] = '0'
+    with mock.patch.dict(os.environ, {'DUMB_INIT_DEBUG': '0'}):
+        yield
 
 
-@pytest.fixture(params=['1', '0'])
+@pytest.yield_fixture(params=['1', '0'])
 def both_setsid_modes(request):
-    os.environ['DUMB_INIT_SETSID'] = request.param
+    with mock.patch.dict(os.environ, {'DUMB_INIT_SETSID': request.param}):
+        yield
 
 
-@pytest.fixture
+@pytest.yield_fixture
 def setsid_enabled():
-    os.environ['DUMB_INIT_SETSID'] = '1'
+    with mock.patch.dict(os.environ, {'DUMB_INIT_SETSID': '1'}):
+        yield
 
 
-@pytest.fixture
+@pytest.yield_fixture
 def setsid_disabled():
-    os.environ['DUMB_INIT_SETSID'] = '0'
+    with mock.patch.dict(os.environ, {'DUMB_INIT_SETSID': '0'}):
+        yield