diff --git a/prometheus_client/multiprocess.py b/prometheus_client/multiprocess.py
index 2fa97168..a45d1ec7 100644
--- a/prometheus_client/multiprocess.py
+++ b/prometheus_client/multiprocess.py
@@ -12,6 +12,8 @@
 class MultiProcessCollector(object):
     """Collector for files for multi-process mode."""
     def __init__(self, registry, path=os.environ.get('prometheus_multiproc_dir')):
+        if not path or not os.path.isdir(path):
+            raise ValueError('env prometheus_multiproc_dir is not set or not a directory')
         self._path = path
         if registry:
           registry.register(self)
diff --git a/tests/test_multiprocess.py b/tests/test_multiprocess.py
index d122dbbf..8f282d18 100644
--- a/tests/test_multiprocess.py
+++ b/tests/test_multiprocess.py
@@ -158,3 +158,28 @@ def test_multi_expansion(self):
 
     def tearDown(self):
         os.unlink(self.tempfile)
+
+class TestUnsetEnv(unittest.TestCase):
+    def setUp(self):
+        self.registry = CollectorRegistry()
+        fp, self.tmpfl = tempfile.mkstemp()
+        os.close(fp)
+
+    def test_unset_syncdir_env(self):
+        self.assertRaises(
+            ValueError,
+            MultiProcessCollector,
+            self.registry
+        )
+
+    def test_file_syncpath(self):
+        registry = CollectorRegistry()
+        self.assertRaises(
+            ValueError,
+            MultiProcessCollector,
+            registry,
+            self.tmpfl
+        )
+
+    def tearDown(self):
+        os.remove(self.tmpfl)