Angular 6, Template parse Exception

This is my Angular code.

<input type="checkbox"  [checked]="fields.check3?.5 != undefined && fields.check3?.5 =='on' ">

At runtime, the browser is throwing the following exception.

Uncaught Error: Template parse errors:
Parser Error: Unexpected token 5, expected identifier or keyword at column 28 in [fields.check3?.'5' != undefined

My data is like this:

"check3": {
                "5": "on"
            }

Can I not use a number as a property? How can i use the same data?

Thank you