Why does button in movieclip produce Error #1034

I have a sliding menu with multiple movieclips.  I want to add buttons to each panel to link to PDFs.  The button works, but the code produces the following error:

TypeError: Error #1034: Type Coercion failed: cannot convert flash.display::SimpleButton@114e2941 to flash.display.MovieClip.

  at slideshow_fla::MainTimeline/onClick()

I tried to bypass the panel tweening portion by checking for child type, but it doesn't seem to ignore the button.  What am I missing?

import gs.*;

import gs.easing.*;

s1.props = {lx:0, rx:350, ind:1};

s2.props = {lx:36.45, rx:386.45, ind:2};

s3.props = {lx:72.90, rx:422.95, ind:3};

s4.props = {lx:109.40, rx:459.45, ind:4};

s5.props = {lx:145.85, rx:495.90, ind:5};

s6.props = {lx:182.35, rx:532.40, ind:6};

s7.props = {lx:218.80, rx:568.85, ind:7};

s8.props = {lx:255.25, rx:605.30, ind:8};

s9.props = {lx:291.75, rx:641.80, ind:9};

s10.props = {lx:328.20, rx:678.25, ind:10};

s11.props = {lx:364.70, rx:714.45, ind:11};

s12.props = {lx:401.15, rx:751.20, ind:12};

s13.props = {lx:437.60, rx:787.65, ind:13};

s14.props = {lx:474.10, rx:824.15, ind:14};

s15.props = {lx:510.55, rx:860.60, ind:15};

s16.props = {lx:547.05, rx:897.10, ind:16};

s17.props = {lx:583.50, rx:933.55, ind:17};

s1.addEventListener(MouseEvent.CLICK, onClick);

s2.addEventListener(MouseEvent.CLICK, onClick);

s3.addEventListener(MouseEvent.CLICK, onClick);

s4.addEventListener(MouseEvent.CLICK, onClick);

s5.addEventListener(MouseEvent.CLICK, onClick);

s6.addEventListener(MouseEvent.CLICK, onClick);

s7.addEventListener(MouseEvent.CLICK, onClick);

s8.addEventListener(MouseEvent.CLICK, onClick);

s9.addEventListener(MouseEvent.CLICK, onClick);

s10.addEventListener(MouseEvent.CLICK, onClick);

s11.addEventListener(MouseEvent.CLICK, onClick);

s12.addEventListener(MouseEvent.CLICK, onClick);

s13.addEventListener(MouseEvent.CLICK, onClick);

s14.addEventListener(MouseEvent.CLICK, onClick);

s15.addEventListener(MouseEvent.CLICK, onClick);

s16.addEventListener(MouseEvent.CLICK, onClick);

s17.addEventListener(MouseEvent.CLICK, onClick);

function onClick(e:MouseEvent):void

{

  var clicked:MovieClip = MovieClip(e.target);

  for(var i:int=0; i<numChildren; i++)

  {

  if(getChildAt(i) is MovieClip){

  var mc:MovieClip = MovieClip(getChildAt(i));

  if(mc.props.ind <= clicked.props.ind)

  TweenLite.to(mc, 1, {x:mc.props.lx, ease:Expo.easeOut});

  else

  TweenLite.to(mc, 1, {x:mc.props.rx, ease:Expo.easeOut});

  }

  }

}

s1.b1.addEventListener(MouseEvent.CLICK, onBtn1Press);

function onBtn1Press(e:MouseEvent)

{

  trace("Button 1 pressed");

}