[Haskell-beginners] my own last function

Hello everyone, I have just started studying and started trying to solve 99 questions in haskell.org. However I got stuck even in the first question. The first question wants me to write my own last function and I wrote this :  q1mylast :: [x] -> x q1mylast [a] = if length [a] <=1 then  a else  q1mylast (tail [a]) This program doesn't work and it gives this error :  Program error: pattern match failure: q1mylast "doaltan" I'll be very glad if you could tell me what's wrong with the code and how can I fix it.. Thanks in advance :)