diff --git a/vine/synchronization.py b/vine/synchronization.py
index 77c80a3..539a5fc 100644
--- a/vine/synchronization.py
+++ b/vine/synchronization.py
@@ -32,6 +32,13 @@ def all_done():
     Note that you cannot add new promises to a barrier after
     the barrier is fulfilled.
     """
+    __slots__ = (
+        'p', 'args', 'kwargs', '_value', 'size',
+        'ready', 'reason', 'cancelled', 'finalized',
+        '__weakref__',
+        # adding '__dict__' to get dynamic assignment
+        "__dict__",
+    )
 
     def __init__(self, promises=None, args=None, kwargs=None,
                  callback=None, size=None):
@@ -57,14 +64,6 @@ def __init__(self, promises=None, args=None, kwargs=None,
         if callback:
             self.then(callback)
 
-        __slots__ = (  # noqa
-            'p', 'args', 'kwargs', '_value', 'size',
-            'ready', 'reason', 'cancelled', 'finalized',
-            '__weakref__',
-            # adding '__dict__' to get dynamic assignment
-            "__dict__",
-        )
-
     def __call__(self, *args, **kwargs):
         if not self.ready and not self.cancelled:
             self._value += 1