document.getElementById('canvas_main')获取绘图上下文总是失败

相关代码如下：
function DrawSpecialClass(spclBodyInfList){
  this.ud = new Object();
  this.ud.pgraph = [ ];  
  this.ud.pdex = [ ];
  this.ud.pindex = [ ];
  this.ud.pvex = [ ];
  this.ud.pval = [ ];
  this.ud.mainCanvas;
  this.ud.mainCt;
}
DrawSpecialClass.prototype.Initpage=function(){

this.ud.mainCanvas = document.getElementById('canvas_main');
this.ud.mainCt=this.ud.mainCanvas.getContext('2d');
this.ud.mainCt.strokeStyle = '#000';
this.ud.mainCt.clearRect(0,0,this.canvasWidth,this.canvasHeight);

};

var DrawSpl=new DrawSpecialClass(spclBodyInfList);
DrawSpl.Initpage();

且已经定义画布如下：
<canvas id="canvas_main" width="1200" height="600" style="border:10px solid #97694F;"></canvas>

提示错误：
this.ud.mainCanvas is null
[在此错误处中断] this.ud.mainCt=this.ud.mainCanvas.getContext('2d');