Pfthb

I'm trying to fit a curve with SimpleCurveFitter of commons.math3.fitting in Scala but I catch an exception :

org.apache.commons.math3.exception.ConvergenceException : Unable to permorm 
 Qr decomposition on jacobian

However, I have checked my gradient calculations.... I still don't see why the exception is raised.
See the code by yourself

 def main(args: Array[String]): Unit = 
 var xv: DenseVector[Double] = linspace(0, 3, 300)
 var yv: DenseVector[Double] = DenseVector.zeros(300)


 for (i <- xv.findAll(x => x < 1.0)) yv.update(i, 1)
 for (i <- xv.findAll(x => x >= 1.0)) yv.update(i, exp(-(xv(i) - 1.0)/1))

 val wop: Array[WeightedObservedPoint] = new Array[WeightedObservedPoint](xv.length)

 for (i <- 0 to xv.length - 1) wop.update(i, new WeightedObservedPoint(1, xv(i), yv(i)))

 val f: ParametricUnivariateFunction = new ParametricUnivariateFunction 
 override def value(x: Double, parameters: Double*): Double = 
 val a = parameters(0)
 val b = parameters(1)

 1.0 / (1.0 + a * pow(x, 2 * b))
 

 override def gradient(x: Double, parameters: Double*): Array[Double] = 
 val a = parameters(0)
 val b = parameters(1)
 val ga = - pow(x, 2 * b) / pow(1 + a * pow(x, 2 * b), 2)
 val gb = - (2 * a * pow(x, 2 * b) * log(x)) / pow(1 + a * pow(x, 2 * b), 2)
 val grad = Array(ga, gb)
 grad
 
 

 val wopc = JavaConverters.asJavaCollection(wop)
 val cf = SimpleCurveFitter.create(f, Array(1, 1))
 val param = cf.fit(wopc)
 println(param(0), param(1))

Thank you for your help :)