Error 422 by posting a new tree

Hi
I am having an issue by posting a new tree on my Github. I wonder if it is my json file:
{"tree": [{
"path": "graphics",
"mode": "040000",
"type": "tree"
},
{
"path": "locale",
"mode": "040000",
"type": "tree"
},
{
"path": "methods",
"mode": "040000",
"type": "tree"
},
{
"path": "migrations",
"mode": "040000",
"type": "tree"
},
{
"path": "prototypes",
"mode": "040000",
"type": "tree"
}]}
, the url I use: https://api.github.com/repos/MyName/MyRepos/git/trees
So, I have this (422) Unprocessable Entity issue, and no further info.
Can you help?
Many thanks
Niko