diff --git a/pymbolic/mapper/__init__.py b/pymbolic/mapper/__init__.py
index 26e88a4d505ed5cc164ec4dbdb9f01ed1106aa19..396dcd2973e0355c4e8b6f7e66f8df5fcffdb103 100644
--- a/pymbolic/mapper/__init__.py
+++ b/pymbolic/mapper/__init__.py
@@ -242,13 +242,10 @@ class NonrecursiveIdentityMapper(IdentityMapperBase, Mapper):
 
 class CSECachingMapperMixin(object):
     def map_common_subexpression(self, expr):
-        from pymbolic.primitives import is_zero
-
         try:
             ccd = self._cse_cache_dict
         except AttributeError:
-            from weakref import WeakKeyDictionary
-            ccd = self._cse_cache_dict = WeakKeyDictionary()
+            ccd = self._cse_cache_dict = {}
 
         try:
             return ccd[expr]