[Clarification] Custom Data Source API: Type of the keys object in /select response

I'm wondering whether this is a typo in the documentation for the /select call. The keys object is described as having string values, but as keys is meant to represent a tuple of members, and member's values can be strings or numbers (in the case that dimensions are of type number or date) would that then not actually make keys a { (uniqueName): string | number } object?
Ref: https://www.flexmonster.com/api/select-request-for-pivot-table/
The examples in that page point to one where there is a numerical dimension listed in the by object, which would have resulted in number-valued members in the keys, but the example is just listed as having the same format as the previous one. 

{
    "aggs"[]: {
        "values": {
            (uniqueName): {
                (func): number
            }
        },
        "keys": {
            (uniqueName): string
        }
    },
    "page": number,
    "pageTotal": number
}