diff --git a/prometheus_client/multiprocess.py b/prometheus_client/multiprocess.py
index 6d8f3445..9a5fa484 100644
--- a/prometheus_client/multiprocess.py
+++ b/prometheus_client/multiprocess.py
@@ -11,13 +11,13 @@
 
 class MultiProcessCollector(object):
     """Collector for files for multi-process mode."""
-    def __init__(self, registry, path=None):
+    def __init__(self, registry=None, path=None):
         if path is None:
             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:
+        if registry is not None:
             registry.register(self)
 
     def collect(self):