diff --git a/moto/dynamodb2/models.py b/moto/dynamodb2/models.py
index 747fa93a7..7d995486d 100644
--- a/moto/dynamodb2/models.py
+++ b/moto/dynamodb2/models.py
@@ -450,7 +450,11 @@ 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(self._get_default(old_list_key))
+                    old_list = self._get_default(old_list_key)
+                    if not isinstance(old_list, DynamoType):
+                        old_list = DynamoType(
+                            expression_attribute_values[old_list]
+                        )
                 else:
                     old_list = self.attrs[old_list_key.split(".")[0]]
                     if "." in old_list_key: