Access array index Yup conditional validation

I want to use YUP conditional validation where the value of element answer[0] is used to see if the element ratio[0] is mandatory or not.

I am having trouble accessing the index [0] of answer to do that validation.

Any ideas? (I wrote answer.0 but it did not work).

 explosion: Yup.object({
    ratio: Yup.object({
      0: Yup.number()
        .typeError("value between 0 and 1")
        .min(0, "Value between 0 and 1")
        .max(1, "Value between 0 and 1")
         Y.when("answer.0", {
          is: "no",
          then: (s) => s.notRequired(),
          otherwise: (s) => s.required("Explosion probability"),
        }),
    }),

    answer: Yup.object({
      0: Yup.string().required("Mandatory field"),
    }),
  }),