diff --git a/setup.py b/setup.py
index c21f3b923a7c79afe9cd4c1f6d8bf0bdfb6aef3d..400d1b9fc1d135b4a4a75dda113cf0af25d7c5cc 100644
--- a/setup.py
+++ b/setup.py
@@ -41,7 +41,7 @@ setup(name="sumpy",
 
       install_requires=[
           "loo.py>=2017.2",
-          "pytools>=2013.5.6",
+          "pytools>=2017.6",
           "boxtree>=2013.1",
           "pytest>=2.3",
           "six",
diff --git a/sumpy/__init__.py b/sumpy/__init__.py
index c770e802e8bab11241b3f9593e5227ab7cb7d8ba..a325e3d1a90a370f7f35883f3de62f6d90486f7c 100644
--- a/sumpy/__init__.py
+++ b/sumpy/__init__.py
@@ -28,7 +28,7 @@ from sumpy.p2e import P2EFromSingleBox, P2EFromCSR
 from sumpy.e2p import E2PFromSingleBox, E2PFromCSR
 from sumpy.e2e import E2EFromCSR, E2EFromChildren, E2EFromParent
 from sumpy.version import VERSION_TEXT
-from pytools.persistent_dict import PersistentDict
+from pytools.persistent_dict import WriteOncePersistentDict
 
 __all__ = [
     "P2P", "P2PFromCSR",
@@ -37,7 +37,7 @@ __all__ = [
     "E2EFromCSR", "E2EFromChildren", "E2EFromParent"]
 
 
-code_cache = PersistentDict("sumpy-code-cache-v6-"+VERSION_TEXT)
+code_cache = WriteOncePersistentDict("sumpy-code-cache-v6-"+VERSION_TEXT)
 
 
 # {{{ cache control
diff --git a/sumpy/tools.py b/sumpy/tools.py
index 12f385f09d79f8e67a14be7753c5c7782efdbab1..428fa2a1ba54afc1151b7874aa89bc81c2bdec35 100644
--- a/sumpy/tools.py
+++ b/sumpy/tools.py
@@ -394,7 +394,7 @@ class KernelCacheWrapper(object):
             knl = self.get_optimized_kernel(**kwargs)
 
         if CACHING_ENABLED:
-            code_cache[cache_key] = knl
+            code_cache.store_if_not_present(cache_key, knl)
 
         return knl