diff --git a/synapse/util/caches/lrucache.py b/synapse/util/caches/lrucache.py
index 570bcac1fd..176011faad 100644
--- a/synapse/util/caches/lrucache.py
+++ b/synapse/util/caches/lrucache.py
@@ -68,7 +68,7 @@ class _Node:
         self.callbacks = callbacks
 
         if asizeof:
-            self.memory = asizeof.asizeof(self)
+            self.memory = asizeof.asizeof(key) + asizeof.asizeof(value)
         else:
             self.memory = 0