'selectionStart' property exists (?) but cannot be accessed

In my simple bookmarklet, I call all `input` elements of the document, and then try to try to access `selectionStart` of each element: ``` javascript: (function () { var inps=document.getElementsByTagName('input'); for (var i = 0; i < inps.length; i++) { var el = inps[i]; if ('selectionStart' in el) { console.log("o: " + (typeof el)); console.log("x: " + (typeof el.nonexistent)); console.log("s: " + (typeof el.selectionStart)); } } })(); ``` this code gives the following lines in the console: "o: object" - as expected, "x: undefined" - as expected, but for `el.selectionStart` no output is given and "NS_ERROR_FAILURE" is shown in the console. Can anybody explain why does this happen? (additional general question - where can I find the meaning or any details about such errors thrown by Firefox?)