I´m at the very beginning with Ruby and give again and again this error

program.rb:6:in `gsub': broken utf-8 string (argumenterror)

when I'm trying this short code:

#coding:utf-8

temp=""
txtfile=File.open("8-3_tiedosto.txt","r");txtfile.each{|row|temp=temp+row};txtfile.close

temp = temp.gsub("", '')
puts temp

The original text in the file contains characters that I do not to
include to my final result, that should only contain ASCII 65..90 and
97..122. So I do not understand, what arguments should be given to gsub?

I'm sorry because of my stupidity :)