diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index 3a0c75ebf2..74ae9cab8e 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -417,6 +417,8 @@ class BaseExpression:
     def get_refs(self):
         refs = set()
         for expr in self.get_source_expressions():
+            if expr is None:
+                continue
             refs |= expr.get_refs()
         return refs