My Longman dictionary states that the comparative of 'shy' is 'shyer'. However, at least two online dictionaries also give the form 'shier' as being acceptable: The Free Dictionary and Merriam-Webster. On an English language forum I came across a reference to British (shyer) vs. American (shier) spelling. But an Ngram chart shows that even in American English 'shyer' is much more used.
My problem is that I've been told that it is definitely wrong, but if it's in dictionaries then... has there been a change to what is wrong?
P.S.: Google Ngram link