code: func (args Arguments) Error(index int) error {
  	obj := args.Get(index)
  	var s error
  	var ok bool
  	if obj == nil {
  		return nil
  	}
  	if s, ok = obj.(error); !ok {
  		panic(fmt.Sprintf("assert: arguments: Error(%d) failed because object wasn't correct type: %v", index, args.Get(index)))
  	}
  	return s
  }
docs: // Error gets the argument at the specified index. Panics if there is no argument, or
  // if the argument is of the wrong type.