diff --git a/pyop2/base.py b/pyop2/base.py
index 374f23b13..b892e2c8b 100644
--- a/pyop2/base.py
+++ b/pyop2/base.py
@@ -2652,15 +2652,15 @@ def _cache_key(cls, maps):
 
     @cached_property
     def _kernel_args_(self):
-        return tuple(itertools.chain(*(m._kernel_args_ for m in self)))
+        return tuple(itertools.chain(*(m._kernel_args_ for m in self if m is not None)))
 
     @cached_property
     def _argtypes_(self):
-        return tuple(itertools.chain(*(m._argtypes_ for m in self)))
+        return tuple(itertools.chain(*(m._argtypes_ for m in self if m is not None)))
 
     @cached_property
     def _wrapper_cache_key_(self):
-        return tuple(m._wrapper_cache_key_ for m in self)
+        return tuple(m._wrapper_cache_key_ for m in self if m is not None)
 
     @cached_property
     def split(self):