compiler error

Error:(33, 48) type mismatch;
 found   : a.type (with underlying type A)
 required: ?{def +: ?}
Note that implicit conversions are not applicable because they are ambiguous:
 both method any2stringadd in object Predef of type [A](self: A)any2stringadd[A]
 and method additiveSemigroupOps in trait AdditiveSemigroupSyntax of type [A](a: A)(implicit evidence$8: spire.algebra.AdditiveSemigroup[A])spire.syntax.AdditiveSemigroupOps[A]
 are possible conversion functions from a.type to ?{def +: ?}
      case _ => Some(vals.reduceLeft((a, b) => a + (b / a)))
                                               ^