SortedDictionary ArgumentException: "Æ" and "AE" considered the same keys

I was trying to use SortedDictionary to store some of my data from file, but got really weird bunch of key duplication exceptions. I came up with the next code sample which reproduces my issue:

var dict = new SortedDictionary<string, string>();
dict.Add("Æ", "qwerty"); // "aesc" (aka "ash"), single symbol
Console.WriteLine(dict["AE"]); // outputs "qwerty" for two-symbol string "AE"
dict.Add("AE", ""); // ArgumentException: An entry with the same key already exists.

This code on .NET Fiddle

It’s not happening for usual Dictionary though and I finally decided to use it instead. But I still wonder why is it a problem for a sorted one? Unfortunatelly, I was unable to google an answer myself (got a lot of AES-related noise) and can’t debug into SortedDictionary’s code despite MS recently made some of .NET source code open.

It’s almost seems like this class implicitly runs some string preprocessing/normalization function, but I just can’t believe it is an actual reason.

Any ideas why is it happening? Thanks in advance!

Source: .net

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