具有计数但无法访问项的集合

tell application "Microsoft PowerPoint"
    set sel to shape range of selection of active window

    count of sel's shapes -- returns 2 for specific case
    -- class of sel's shapes -- throws a compilation error "object you are trying to access does not exist"
    set i to item 1 of sel's shapes -- i not set but this line does not throw an error
    i -- error: the variable i is not defined
end tell

注释指示运行特定行时发生的情况。有趣的是, sel's shapes 确实有计数,但我无法从中提取任何项目。我的第一直觉是 塞尔的形状 必须是其他数据类型,但 class sel's shapes 还抛出一个错误,抱怨 塞尔的形状

问:这里发生了什么?怎么可能 count of 定义(和工作!),同时 item 1 of class of 不是吗?