Uncaught TypeError: Cannot set property 'onmouseover' of undefined

<script>
      function getByClass(oParent, sClass) {
        var aEle = oParent.getElementsByTagName("*");
        var aResult = [];

        for (var i = 0; i < aEle.length; i++) {
          if (aEle[i].className == sClass) {
            aResult.push(aEle[i]);
          }
        }
        return aResult;
      }

      var oDiv = document.getElementById("playimages");
      var oBtnPrev = getByClass(oDiv, "prev")[0];
      var oBtnNext = getByClass(oDiv, "next")[0];
      var oMarkLeft = getByClass(oDiv, "mark_left")[0];
      var oMarkRight = getByClass(oDiv, "mark_rignt")[0];

      //左右按钮
      oBtnPrev.onmouseover = oMarkLeft.onmouseover = function(){
        startMove(oBtnPrev,"opacity",100);
      }
      oBtnPrev.onmouseout = oMarkLeft.onmouseout = function(){
        startMove(oBtnPrev,"opacity",0);
      }

      oBtnNext.onmouseover = oMarkRight.onmouseover = function(){
        startMove(oBtnNext,"opacity",100);
      }
      oBtnNext.onmouseout = oMarkRight.onmouseout = function(){
        startMove(oBtnNext,"opacity",0);
      }

给oBtnPrev添加onmouseover就能正常显示
但是给oBtnNext添加omouseover就会显示这个错误
请问这个应该怎么解决呢