why this compile error?

mapaccuml.idr

mapAccumL : Traversable t => (a -> b -> (c, a)) -> a -> t b -> (t c, a)
mapAccumL f a1 tb = runState (traverse stateFlip tb) a1
  where
    -- if I comment next line, it compiles OK
    -- else it fails with :
    -- 
    --     test.idr:91:37:When checking right hand side of test.mapAccumL, stateFlip with expected type
    --         StateT a Identity c

    -- When checking an application of f:
    --         Type mismatch between
    --                 b1 (Type of b1)
    --         and
    --                 b (Expected type)
    stateFlip : b -> StateT a Identity c
    stateFlip b1 = ST (\x => pure(f x b1))