Storyboard Animation Error

//address the storyboard by its name
           Storyboard sb = (Storyboard)this.FindResource("RectangleAnimation");

           //search for the two DoubleAnimationUsingKeyFrames elements in the storyboard
           DoubleAnimationUsingKeyFrames translate = (DoubleAnimationUsingKeyFrames)sb.Children[0];
           DoubleAnimationUsingKeyFrames rotate = (DoubleAnimationUsingKeyFrames)sb.Children[1];

           //search for the two splinedoublekeyframe elements in the storyboard
           SplineDoubleKeyFrame translateFrame = (SplineDoubleKeyFrame)translate.KeyFrames[0];
           SplineDoubleKeyFrame rotateFrame = (SplineDoubleKeyFrame)rotate.KeyFrames[0];

           //duration: adjust the property keytime to the value of the slider sldDuration
           translateFrame.KeyTime = new TimeSpan(0, 0, (int)sldDuration.Value);
           rotateFrame.KeyTime = new TimeSpan(0, 0, (int)sldDuration.Value);

           //rotation: adjust the property Value of the rotateFrame to he value of te lsider sldrotation
           rotateFrame.Value = sldRotation.Value;

           //distance:adjust the property value of the translateFrame to the value of the slider sldDistance
           translateFrame.Value = sldDistance.Value;

           //finally start the animation
           sb.Begin(this);

 SplineDoubleKeyFrame translateFrame = (SplineDoubleKeyFrame)translate.KeyFrames[0];

saying: Unable to cast object of type 'System.Windows.Media.Animation.EasingDoubleKeyFrame' to type 'System.Windows.Media.Animation.SplineDoubleKeyFrame'.