diff --git a/sympy/core/symbol.py b/sympy/core/symbol.py
index eb8d802e1c..d8e111ed77 100644
--- a/sympy/core/symbol.py
+++ b/sympy/core/symbol.py
@@ -34,7 +34,7 @@ class Symbol(AtomicExpr, Boolean):
 
     is_comparable = False
 
-    __slots__ = ['name']
+    __slots__ = ['name', '__weakref__']
 
     is_Symbol = True