SliderExtender problem with property xmlns:ajaxtoolkit

I am using a slider extender control.  Every time I build, this property is automatically added to the slider extender properties by Visual Studio 2013:

xmlns:ajaxtoolkit="ajaxcontroltoolkit"

Then it gives me an error "Type 'AjaxControlToolkit.SliderExtender' does not have a public property named 'xmlns:ajaxtoolkit'."

I manually delete all of them (I have about 20 sliders!)...Then as soon as re-build, they're BACK!!!

I registered AjaxControlToolkit tag prefix in my web config file:

<addtagPrefix="ajaxToolkit"assembly="AjaxControlToolkit"namespace="AjaxControlToolkit" />

What is going on?

(BTW, somebody ought to improve this text area.  It's a pain to  type or paste anything in here.)

Appreciate any assistance