javax.servlet.ServletException: Cannot find bean under

I get the following error when trying to use el to assign value to collection 
attribute in struts's html:options:
javax.servlet.ServletException: Cannot find bean under
name 
[com.my.DAO.AttrColorDTO@10eeb26,com.my.DAO.AttrColorDTO@704cf5,com.my.DAO.AttrColorDTO@cc7f9e,com.my.DAO.AttrColorDTO@1ce5e7a,com.my.DAO.AttrColorDTO@19cd5f5]

Below is what the struts html:options looks like:
<html:select property="color" >
  <html:options collection="${jspHelper.attrList.attrColorDTOList}" 
property="ID" labelProperty="shortDesc"/>
</html:select>



However, it work when I do the following:
1) save the attrColorDTOList to a session:  
session.setAttribute("attrColorDTOList" 
jspHelper.getAttrList().getAttrColorDTOList)

2) In jsp:
<html:select property="color" >
  <html:options collection="attrColorDTOList"  property="ID" 
labelProperty="shortDesc"/>
</html:select>


Any idea why the first method is not working?