diff --git a/moto/dynamodb2/models.py b/moto/dynamodb2/models.py
index 88f750775..747fa93a7 100644
--- a/moto/dynamodb2/models.py
+++ b/moto/dynamodb2/models.py
@@ -450,9 +450,7 @@ class Item(BaseModel):
                 old_list_key = list_append_re.group(1)
                 # old_key could be a function itself (if_not_exists)
                 if old_list_key.startswith("if_not_exists"):
-                    old_list = DynamoType(
-                        expression_attribute_values[self._get_default(old_list_key)]
-                    )
+                    old_list = DynamoType(self._get_default(old_list_key))
                 else:
                     old_list = self.attrs[old_list_key.split(".")[0]]
                     if "." in old_list_key: