I dont know why but a custom character wasnt rendered

I dont know why but a custom character wasnt rendered... 😢 
I declare this character with this in my default.json :
{
            "type": "bitmap",
            "file": "minecraft:font/title.png",
            "ascent": 8,
            "height": 8,
            "chars": [
                "+"
            ]
        }

But doesnt work