diff --git a/Lib/_weakrefset.py b/Lib/_weakrefset.py
index 4265369..c5de222 100644
--- a/Lib/_weakrefset.py
+++ b/Lib/_weakrefset.py
@@ -63,7 +63,8 @@ class WeakSet:
                     yield item
 
     def __len__(self):
-        return sum(x() is not None for x in self.data)
+        with _IterationGuard(self):
+            return sum(x() is not None for x in self.data)
 
     def __contains__(self, item):
         try: