diff --git a/pyop2/caching.py b/pyop2/caching.py
index 7b0b85735..24a3f5513 100644
--- a/pyop2/caching.py
+++ b/pyop2/caching.py
@@ -273,9 +273,10 @@ def decorator(func):
         def wrapper(*args, **kwargs):
             if collective:
                 comm, disk_key = key(*args, **kwargs)
+                disk_key = _as_hexdigest(disk_key)
                 k = hash_comm(comm), disk_key
             else:
-                k = key(*args, **kwargs)
+                k = _as_hexdigest(key(*args, **kwargs))
 
             # first try the in-memory cache
             try:
@@ -286,40 +287,59 @@ def wrapper(*args, **kwargs):
             # then try to retrieve from disk
             if collective:
                 if comm.rank == 0:
-                    v = _disk_cache_setdefault(cachedir, disk_key, lambda: func(*args, **kwargs))
+                    v = _disk_cache_get(cachedir, disk_key)
                     comm.bcast(v, root=0)
                 else:
                     v = comm.bcast(None, root=0)
             else:
-                v = _disk_cache_setdefault(cachedir, k, lambda: func(*args, **kwargs))
+                v = _disk_cache_get(cachedir, k)
+            if v is not None:
+                return cache.setdefault(k, v)
+
+            # if all else fails call func and populate the caches
+            v = func(*args, **kwargs)
+            if collective:
+                if comm.rank == 0:
+                    _disk_cache_set(cachedir, disk_key, v)
+            else:
+                _disk_cache_set(cachedir, k, v)
             return cache.setdefault(k, v)
         return wrapper
     return decorator
 
 
-def _disk_cache_setdefault(cachedir, key, default):
-    """If ``key`` is in cache, return it. If not, store ``default`` in the cache
-    and return it.
+def _as_hexdigest(key):
+    return hashlib.md5(str(key).encode()).hexdigest()
 
-    :arg cachedir: The cache directory.
-    :arg key: The cache key.
-    :arg default: Lazily evaluated callable that returns a new value to insert into the cache.
 
-    :returns: The value associated with ``key``.
-    """
-    key = hashlib.md5(str(key).encode()).hexdigest()
-    key1, key2 = key[:2], key[2:]
+def _disk_cache_get(cachedir, key):
+    """Retrieve a value from the disk cache.
 
-    basedir = Path(cachedir, key1)
-    filepath = basedir.joinpath(key2)
+    :arg cachedir: The cache directory.
+    :arg key: The cache key (must be a string).
+    :returns: The cached object if found, else ``None``.
+    """
+    filepath = Path(cachedir, key[:2], key[2:])
     try:
         with open(filepath, "rb") as f:
             return pickle.load(f)
     except FileNotFoundError:
-        basedir.mkdir(parents=True, exist_ok=True)
-        tempfile = basedir.joinpath(f"{key2}_p{os.getpid()}.tmp")
-        obj = default()
-        with open(tempfile, "wb") as f:
-            pickle.dump(obj, f)
-        tempfile.rename(filepath)
-        return obj
+        return None
+
+
+def _disk_cache_set(cachedir, key, value):
+    """Store a new value in the disk cache.
+
+    :arg cachedir: The cache directory.
+    :arg key: The cache key (must be a string).
+    :arg value: The new item to store in the cache.
+    """
+    k1, k2 = key[:2], key[2:]
+    basedir = Path(cachedir, k1)
+    basedir.mkdir(parents=True, exist_ok=True)
+
+    tempfile = basedir.joinpath(f"{k2}_p{os.getpid()}.tmp")
+    filepath = basedir.joinpath(k2)
+    with open(tempfile, "wb") as f:
+        pickle.dump(value, f)
+    tempfile.rename(filepath)