Undefined offset: 245 phpmyadmin when inserting json

I have the following json in a mysql column

{
    "links_normales": [{
        "item_submenu": "01"
    }, {
        "item_submenu": "02"
    }]
}

When I enter I get Some errors were detected on the server! Check the footer of this window.

And below Notice in ./libraries/dbi/DBIMysqli.class.php#522  Undefined offset: 245

I have: Server type: MySQL Server version: 5.7.13-0ubuntu0.16.04.2 - (Ubuntu)