Strange Behavior with Schema.struct Declaration

declare const basis: string;
/*
Schema.Schema<never, {
    readonly [x: string]: number;
    readonly [x: number]: number;
}>

instead of

Schema.Schema<never, {
    readonly [x: string]: number;
}>

*/
const tryIt = Schema.struct({ [basis]: Schema.number });

declare const basis: string;
/*
Schema.Schema<never, {
    readonly [x: string]: number;
    readonly [x: number]: number;
}>

instead of

Schema.Schema<never, {
    readonly [x: string]: number;
}>

*/
const tryIt = Schema.struct({ [basis]: Schema.number });