Passing specs for the Fixnum#stringify exercise in RSPEC 3 section

I apologize ahead of time if this is just my ignorance on this topic. I successfully coded the base converter Fixnum#stringify as per the instructions in the last exercise of the RSPEC 3 section. My only question is that, to get it to pass specs, I had to use String#downcase so as to make sure to convert alpha digits (such as “A” => “a”) to get it to pass specs. However, the instructions require a hash to be built with 0-15 as keys and “1”…“9” then “A”…“F” as values for the purposes of hexadecimal conversion - which is how I built it. So if I am converting a digit 10 with this hash like so base_hash[10] I get “A” instead of “a”. But the spec is wanting “a”. Is there some convention I am missing wherein you should always use the lowercase version of a hexadecimal upon conversion?? I have checked base converters and they all say 10 is “A” not “a”, so outside of some convention I am lost as to why the specs are written this way. Is it a mistake in the spec?