After doing some further testing, I found that <s:updownselect list="courseList" name="nonsenseName" listKey="uuid" listValue="displayName"/>
works, but <s:updownselect list="courseList" name="selectedCourses" listKey="uuid" listValue="displayName"/> Does not. selectedCourses is a String array on my action object, and before this issue popped up, I was using OGNL to autopopulate the array with the selected ids from courseList. Now, every variation on "selectedCourses" raises this error. (I've tried selectedCourses 'selectedCourses' %{selectedCourses} %{'selectedCourses'} so far). -Brian