Could not set post labels using Blogger v3 API?

Dear all,
I have tried to insert new post with Blogger v3 API using APIs Explorer 

https://developers.google.com/blogger/docs/3.0/reference/posts/insert

It has been successfully inserted when I did not set labels, but it always 
gave the error below when I tried to set labels even one label only.

Is this a bug? 
Or did I miss something?
Request

POST https://www.googleapis.com/blogger/v3/blogs/###/posts?fields=id%2Curl&key={YOUR_API_KEY}

{
 "labels": [
  "test"
 ],
 "title": "test",
 "author": {
  "id": "###"
 },
 "content": "testing"
}

Response

400 OK

{
 "error": {
  "errors": [
   {
    "domain": "global",
    "reason": "invalid",
    "message": "We're sorry, but the value for field labels was not valid."
   }
  ],
  "code": 400,
  "message": "We're sorry, but the value for field labels was not valid."
 }
}

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