diff --git a/controller/api/danmaku_clip_blueprint.py b/controller/api/danmaku_clip_blueprint.py
index 9415034..02e721e 100644
--- a/controller/api/danmaku_clip_blueprint.py
+++ b/controller/api/danmaku_clip_blueprint.py
@@ -2,9 +2,9 @@ import os
 
 from flask import Blueprint, jsonify, request
 
+from model import db
 from model.DanmakuClip import DanmakuClip
 from util.flask import not_found_json_response, error_json_response
-from model import db
 
 blueprint = Blueprint("api_danmaku_clip", __name__, url_prefix="/api/danmaku_clip")
 
@@ -52,5 +52,5 @@ def delete_video_clip(danmaku_clip_id):
         db.session.commit()
     return jsonify({
         "id": danmaku_clip_id,
-        "old_data": danmaku_clip
+        "old_data": danmaku_clip.to_json() if danmaku_clip is not None else {}
     })
diff --git a/controller/api/video_clip_blueprint.py b/controller/api/video_clip_blueprint.py
index 252b4cf..05de25c 100644
--- a/controller/api/video_clip_blueprint.py
+++ b/controller/api/video_clip_blueprint.py
@@ -2,10 +2,10 @@ import os
 
 from flask import Blueprint, jsonify, request
 
-from util.flask import not_found_json_response, error_json_response
-from workflow.video import get_video_real_duration, duration_str_to_float
 from model import db
 from model.VideoClip import VideoClip
+from util.flask import not_found_json_response, error_json_response
+from workflow.video import get_video_real_duration, duration_str_to_float
 
 blueprint = Blueprint("api_video_clip", __name__, url_prefix="/api/video_clip")
 
@@ -56,5 +56,5 @@ def delete_video_clip(video_clip_id):
         db.session.commit()
     return jsonify({
         "id": video_clip_id,
-        "old_data": video_clip
+        "old_data": video_clip.to_json() if video_clip is not None else {}
     })
diff --git a/controller/api/workflow_blueprint.py b/controller/api/workflow_blueprint.py
index 5828cab..ec0ba22 100644
--- a/controller/api/workflow_blueprint.py
+++ b/controller/api/workflow_blueprint.py
@@ -2,10 +2,10 @@ import threading
 
 from flask import Blueprint, jsonify
 
+from model import db
 from model.Workflow import Workflow
 from util.flask import not_found_json_response
 from workflow.worker import do_workflow
-from model import db
 
 blueprint = Blueprint("api_workflow", __name__, url_prefix="/api/workflow")
 
@@ -40,7 +40,7 @@ def delete_workflow(workflow_id):
         db.session.commit()
     return jsonify({
         "id": workflow_id,
-        "old_data": workflow
+        "old_data": workflow.to_json() if workflow is not None else {}
     })