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

No repository file, test, or entry point is named. Start by checking the Git trees API requirements against the supplied JSON and URL, then reproduce the 422 response; done means identifying the missing or invalid request detail and documenting or correcting it.

Written by the indexing model from the issue text.