I'm still pretty new to WPF so you can chalk this one up to inexperience. I was trying to make a textbox scale in/scale out based on a trigger monitoring visibility. My thought is I needed to set Storyboard.Target="RenderTransform" and Storyboard.TargetProperty="ScaleX" but doing so got me the above error. The problem is that Target expects a dependency object and RenderTransform is an attached property. The simple fix was just to set Storyboard.TargetProperty="RenderTransform.ScaleX.