Seedanceで「最初/最後のフレームは参照メディアと混在できません」と表示される

The parameter `content` specified in the request is not valid:
first/last frame content cannot be mixed with reference media content.

{
  "model": "doubao-seedance-2-0-260128",
  "prompt": "Replace the person in video 1 with the character in image 1 ... (image 1 is not the first frame) ... do not use first/last frame mode",
  "duration": 14,
  "ratio": "9:16",
  "resolution": "720p",
  "images": ["data:image/jpeg;base64,/9j/4AAQ..."],
  "videos": ["/assets/input/ai_ref_xxxx.mp4"]
}

import os, requests

BASE = "https://api.apiyi.com/seedance/api/v3/contents/generations/tasks"
headers = {"Authorization": f"Bearer {os.environ['APIYI_API_KEY']}"}

body = {
    "model": "doubao-seedance-2-0-260128",
    "content": [
        {"type": "text", "text": "Replace the person in @video1 with the character in @image1, keeping the motion, expressions and background music unchanged"},
        {"type": "image_url", "image_url": {"url": "https://cdn.example.com/character.png"},
         "role": "reference_image"},
        {"type": "video_url", "video_url": {"url": "https://cdn.example.com/source.mp4"},
         "role": "reference_video"},
    ],
    "resolution": "720p",
    "ratio": "9:16",
    "duration": 10,
}

r = requests.post(BASE, headers=headers, json=body, timeout=60)
print(r.status_code, r.json())   # on success: {"id": "cgt-..."}, then poll with that id

curl -s -o /dev/null -w 'code=%{http_code} type=%{content_type} size=%{size_download}\n' '<URL>'