Ruby: string not matched (IndexError)

You probably expected the variable to be a hash, but it is a string.

irb(main):010> x = "123"
=> "123"
irb(main):011> x["12"] = "abc"
=> "abc"
irb(main):012> x
=> "abc3"
irb(main):013> x["12"] = "abc"
(irb):13:in `[]=': string not matched (IndexError)

[]= allows to replace substring with another string, even with a different size.