diff --git a/pixelocr/utils.py b/pixelocr/utils.py
index 4a6c98e..239f10e 100644
--- a/pixelocr/utils.py
+++ b/pixelocr/utils.py
@@ -16,20 +16,18 @@
 
 import functools
 import itertools
+from collections import defaultdict
 from threading import Lock
 
 
 def cached_property(fun):
     """A memoize decorator for class properties."""
     lock = Lock()
-    locks = {}
+    locks = defaultdict(Lock)
     @functools.wraps(fun)
     def get(self):
         with lock:
-            try:
-                obj_lock = self._lock
-            except AttributeError:
-                obj_lock = self._lock = Lock()
+            obj_lock = locks[self, fun]
         with obj_lock:
             try:
                 cache = self._cache