Mutations not returning correct value on one of the test cases

I need some help, this is what i have, for some reason its returning mutation([“hello”, “hey”]) as true, all of the other cases are returning correctly though


function mutation(arr) {
var b = arr[1].toLowerCase();
var a = arr[0].toLowerCase();
for (i=0; i<b.length; i++) {
  if (a.indexOf(b[i]) === -1) {
  return false;
} else{
  return true;
    }
}
}
mutation(["hello", "hey"]);