diff --git a/pixelocr/utils.py b/pixelocr/utils.py
index 239f10e..0253761 100644
--- a/pixelocr/utils.py
+++ b/pixelocr/utils.py
@@ -23,11 +23,14 @@ from threading import Lock
 def cached_property(fun):
     """A memoize decorator for class properties."""
     lock = Lock()
-    locks = defaultdict(Lock)
     @functools.wraps(fun)
     def get(self):
         with lock:
-            obj_lock = locks[self, fun]
+            try:
+                obj_locks = self._locks
+            except AttributeError:
+                obj_locks = self._locks = defaultdict(Lock)
+            obj_lock = obj_locks[fun]
         with obj_lock:
             try:
                 cache = self._cache