Type switch failing?

// when passing in a schema.MultiError
func FormatErrForResp(err error) error {
  fmt.Println(reflect.TypeOf(err)) // prints schema.MultiError

  what, ok := err.(schema.MultiError)
  fmt.Println(what) // prints "0 errors" (zero value for MultiError)
  fmt.Println(ok) // prints false

  switch v := err.(type) {
  case schema.MultiError:
    // do something
  default:
    // do something
  }
}

Reflection shows that it's of typeschema.MultiError but both the type switch and the type assertion are failing. It falls to the default case

Additional information: in the function where the error is first encountered, the type switch goes through the correct case:

if err := schema.NewDecoder().Decode(dst, r.Form); err != nil {
  switch err.(type) {
  case schema.MultiError:
    // passes through here (correct)
  default:
    //
  }
   return err
}

Please put a working sample on the go playground, it will make it easier for us to inspect what you have outside of this function.
Are you sure you're not using a pointer, and the case should be case *schema.MultiError?
Edit: here's my version of a working playground, so I'm betting there's some pointer mixup from what I've seen dealing with type switches.