Hey Guys...need help. Working on a project and get this error on the Output Log

TypeError: Error #1009: Cannot access a property or method of a null object reference. at FlashSite_fla::MainTimeline/frame16()

Here's every frame that is on, begins, or crosses frame 16

Layer Name: Top Menu (4 Button named Home_btn, Works_btn, Tech_btn, Contact_btn)
Code attached to frame:
stop();
Home_btn.addEventListener(MouseEvent.CLICK, home);
function home(event:MouseEvent):void {
gotoAndStop(16);
}
Works_btn.addEventListener(MouseEvent.CLICK, works);
function works(event:MouseEvent):void {
gotoAndStop(17);
}
Tech_btn.addEventListener(MouseEvent.CLICK, tech);
function tech(event:MouseEvent):void {
gotoAndStop(18);
}
Contacts_btn.addEventListener(MouseEvent.CLICK, contact);
function contact(event:MouseEvent):void {
gotoAndStop(19);
}
Layer Name: Investment Opp (button named Invest_btn)
Code attached to frame:
Invest_btn.addEventListener(MouseEvent.CLICK, invest);
function invest(event:MouseEvent):void {
var link:URLRequest = new URLRequest('#');
navigateToURL(link);
}
Layer Name: MfgOpp (Button named Mfg_btn)
Code attached to frame:
Mfg_btn.addEventListener(MouseEvent.CLICK, mfg);
function mfg(event:MouseEvent):void {
var link:URLRequest = new URLRequest('#');
navigateToURL(link);
}
Layer Name: MarketResearch (button name Own_btn)
Code attached to frame:
Own_btn.addEventListener(MouseEvent.CLICK, own);
function own(event:MouseEvent):void {
var link:URLRequest = new URLRequest('#');
navigateToURL(link);
}
Layer Name: ActionScript
Code attached to frame:
import flash.events.MouseEvent;

What am I doing wrong?!?!