diff --git a/pixelocr/utils.py b/pixelocr/utils.py
index 6a85974..4a6c98e 100644
--- a/pixelocr/utils.py
+++ b/pixelocr/utils.py
@@ -16,19 +16,29 @@
 
 import functools
 import itertools
+from threading import Lock
 
 
 def cached_property(fun):
     """A memoize decorator for class properties."""
+    lock = Lock()
+    locks = {}
     @functools.wraps(fun)
     def get(self):
-        try:
-            return self._cache[fun]
-        except AttributeError:
-            self._cache = {}
-        except KeyError:
-            pass
-        ret = self._cache[fun] = fun(self)
+        with lock:
+            try:
+                obj_lock = self._lock
+            except AttributeError:
+                obj_lock = self._lock = Lock()
+        with obj_lock:
+            try:
+                cache = self._cache
+            except AttributeError:
+                cache = self._cache = {}
+            try:
+                ret = cache[fun]
+            except KeyError:
+                ret = cache[fun] = fun(self)
         return ret
     return property(get)