diff --git a/prometheus_client/multiprocess.py b/prometheus_client/multiprocess.py
index 6c953747..514d7784 100644
--- a/prometheus_client/multiprocess.py
+++ b/prometheus_client/multiprocess.py
@@ -156,6 +156,11 @@ def mark_process_dead(pid, path=None):
     """Do bookkeeping for when one process dies in a multi-process setup."""
     if path is None:
         path = os.environ.get('PROMETHEUS_MULTIPROC_DIR', os.environ.get('prometheus_multiproc_dir'))
+
+    if path is None:
+        # Avoid error when environment is not correctly set-up
+        return
+
     for f in glob.glob(os.path.join(path, f'gauge_livesum_{pid}.db')):
         os.remove(f)
     for f in glob.glob(os.path.join(path, f'gauge_liveall_{pid}.db')):
diff --git a/tests/test_multiprocess.py b/tests/test_multiprocess.py
index 9ec0578f..29739997 100644
--- a/tests/test_multiprocess.py
+++ b/tests/test_multiprocess.py
@@ -60,7 +60,7 @@ def _value_class(self):
         return
 
     def tearDown(self):
-        del os.environ['PROMETHEUS_MULTIPROC_DIR']
+        os.environ.pop('PROMETHEUS_MULTIPROC_DIR', None)
         shutil.rmtree(self.tempdir)
         values.ValueClass = MutexValue
 
@@ -303,6 +303,11 @@ def test_missing_gauge_file_during_merge(self):
             os.path.join(self.tempdir, 'gauge_livesum_9999999.db'),
         ]))
 
+    def test_mark_dead_no_env(self):
+        del os.environ['PROMETHEUS_MULTIPROC_DIR']
+
+        mark_process_dead(123)
+
 
 class TestMmapedDict(unittest.TestCase):
     def setUp(self):