compiler error with generics

spire-maths-gist.scala

  def safeSum[A: Numeric](as: Seq[A]):Option[A] =
    if (as.isEmpty) None else Some(as.qsum)

  def safeDiv[A: Numeric](a:Option[A], b:Option[A]):Option[Double] = (a,b) match {
    case (None,_) | (_,Some(0)) | (_,None) => None
    case (Some(x),Some(y)) => Some((x / y).toDouble())
  }

  /**
   * Standard Deviation of the values.
   * Square root of Variance
   * Variance ( The average of the squared differences from the Mean )
   * @param a Sequence of Values (Optionals)
   * @param i Implict Numeric helper
   * @tparam A Our Number Type
   * @return
   */
  def safeStdDev[A: Numeric](a: Seq[A])(implicit i: Numeric[A]):Option[A] = {
    val sum = safeSum[A](a)
    val len = a.size
    safeDiv[A](sum,Some(i.fromInt(len))) match {
      case None => None
      case Some(avg:A) => {
        // squared differences from the average
        val devs = a.map(value => i.times( i.minus(value,avg) , i.minus(value,avg) ) )
        Some(sqrt(devs.reduce((s,a) => i.plus(s,a)) / len))
      }
    }
  }


  /*


  Error:(80, 25) ambiguous implicit values:
 both value i of type spire.math.Numeric[A]
 and value evidence$9 of type spire.math.Numeric[A]
 match expected type spire.math.Numeric[A]
    val sum = safeSum[A](a)
                        ^

   */