Javascript to promote top level lists

I just cannot seem to get this expression to work.  Is purpose is to scan for top-level fields of an object and replaces any single-element array value (list) with just that one value.  I do not want it to recurse.

Here are some examples I am looking for:

Input

{
  "name": ["Alice"],
  "roles": ["admin", "editor"],
  "active": [true],
  "profile": { "city": ["Springfield"] }
}

Output:

{
  "name": "Alice",                    // promoted
  "roles": ["admin", "editor"],       // unchanged
  "active": true,                     // promoted
  "profile": { "city": ["Springfield"] }  // untouched (no recursion)
}

I keep getting: Failure: The output document is a primitive value: null, Reason: The output document must be an array or object, Resolution: Check for target paths that write to the root

What am I missing?

{
  promoteSingleArrays : (obj) => 
    // Promote top-level single-item arrays of primitive types only
    obj
    .mapValues((val, key) => 
      (Array.isArray(val) && val.length == 1 &&
       (typeof val[0] == 'string' || typeof val[0] == 'number' || typeof val[0] == 'boolean'))
        ? val[0]
        : val
    )
}