why am i getting warning in the following code?? though it works perfectly!!!

Prelude> newtype Pair1  c b a=Pair1 {getPair1 :: (a,b,c)}

Prelude> :{
Prelude| instance Functor (Pair1 m n) where
Prelude| fmap f (Pair1 (x,y,z))=Pair1 (f x,y,z)
Prelude| :}

<interactive>:55:10: Warning:
    No explicit implementation for
      ‘Prelude.fmap’
    In the instance declaration for ‘Functor (Pair1 m n)’

-- The below shows it is working fine!
Prelude> getPair1 $ fmap (*100) (Pair1 (2,3,1))
(200,3,1)