diff --git a/src/propcache/_helpers_c.pyx b/src/propcache/_helpers_c.pyx
index 5369e12..a743dc2 100644
--- a/src/propcache/_helpers_c.pyx
+++ b/src/propcache/_helpers_c.pyx
@@ -27,11 +27,10 @@ cdef class under_cached_property:
     def __get__(self, inst, owner):
         if inst is None:
             return self
-        cdef dict cache = inst._cache
-        val = cache.get(self.name, _sentinel)
+        val = (<dict>inst._cache).get(self.name, _sentinel)
         if val is _sentinel:
             val = self.wrapped(inst)
-            cache[self.name] = val
+            (<dict>inst._cache)[self.name] = val
         return val
 
     def __set__(self, inst, value):