请问我这段代码哪里有错误，为什么画不出圆来

html代码

     <div class="step">

          <canvas id="steps" width="440" height="90"></canvas>

     </div>

js代码

    window.onload = function(){

        var canvas = document.getElementById("steps");

        var cxt = canvas.getContext('2d');

        canvas.width =440;

        canvas.height = 90;

        cxt.fillStyle="red";

        cxt.arc(45, 45, 25, 0, 2*Math.PI, false);

        cxt.fill();

}