unflatten object with number as key replaces object with an array of n number length

I have a use case where i always have to unflatten a json object and i dont know if that object is already unflattened. If there is a number for an object key unflatten method create and array which replaces the original object and the array length matches the number plus one. With the last element being the element originally stored by the key. This happens only for nest objects not root level.

example:

flat.unflatten({ testing: { 12: 12 }}) => { testing: [,,,,,,,,,,,,12]}