diff --git a/pyopencl/invoker.py b/pyopencl/invoker.py
index a11c6732e438e0a0021b218b8ab9888e3f62bfde..8cad3f258c0036f24fd5f95e34b3b512d4f61542 100644
--- a/pyopencl/invoker.py
+++ b/pyopencl/invoker.py
@@ -29,7 +29,7 @@ import numpy as np
 
 from warnings import warn
 from pyopencl._cffi import ffi as _ffi
-from pytools.persistent_dict import PersistentDict
+from pytools.persistent_dict import WriteOncePersistentDict
 from pyopencl.tools import _NumpyTypesKeyBuilder
 
 _PYPY = '__pypy__' in sys.builtin_module_names
@@ -358,7 +358,8 @@ def _generate_enqueue_and_set_args_module(function_name,
     return gen.get_picklable_module(), enqueue_name
 
 
-invoker_cache = PersistentDict("pyopencl-invoker-cache-v1",
+invoker_cache = WriteOncePersistentDict(
+        "pyopencl-invoker-cache-v1",
         key_builder=_NumpyTypesKeyBuilder())
 
 
@@ -381,7 +382,7 @@ def generate_enqueue_and_set_args(function_name,
 
     if not from_cache:
         result = _generate_enqueue_and_set_args_module(*cache_key)
-        invoker_cache[cache_key] = result
+        invoker_cache.store_if_not_present(cache_key, result)
 
     pmod, enqueue_name = result
 
diff --git a/pyopencl/scan.py b/pyopencl/scan.py
index 169939cab356e0f1ba7be00e7fa82e19e22979a5..d4226cf60315e237aac3b90001234def4505d92b 100644
--- a/pyopencl/scan.py
+++ b/pyopencl/scan.py
@@ -40,7 +40,7 @@ from pyopencl.tools import (dtype_to_ctype, bitlog2,
 import pyopencl._mymako as mako
 from pyopencl._cluda import CLUDA_PREAMBLE
 
-from pytools.persistent_dict import PersistentDict
+from pytools.persistent_dict import WriteOncePersistentDict
 
 
 import logging
@@ -1132,7 +1132,7 @@ class _GenericScanKernelBase(object):
     # }}}
 
 
-generic_scan_kernel_cache = PersistentDict(
+generic_scan_kernel_cache = WriteOncePersistentDict(
         "pyopencl-generated-scan-kernel-cache-v1",
         key_builder=_NumpyTypesKeyBuilder())
 
@@ -1187,7 +1187,7 @@ class GenericScanKernel(_GenericScanKernelBase):
                       self.second_level_scan_gen_info,
                       self.final_update_gen_info)
 
-            generic_scan_kernel_cache[cache_key] = result
+            generic_scan_kernel_cache.store_if_not_present(cache_key, result)
 
         # Build the kernels.
         self.first_level_scan_info = self.first_level_scan_gen_info.build(
diff --git a/setup.py b/setup.py
index d487cc1c1b73447a2b617962cef24efc46a75a8c..cc286e1f3ab6f548720a68268e846ef7ffede1db 100644
--- a/setup.py
+++ b/setup.py
@@ -225,7 +225,7 @@ def main():
 
             install_requires=[
                 "numpy",
-                "pytools>=2017.2",
+                "pytools>=2017.6",
                 "pytest>=2",
                 "decorator>=3.2.0",
                 "cffi>=1.1.0",