google apps script - OpenByUrl throwing error: You do not have permission to call openByUrl (line 39, file "Code") -

i have script in google spreadsheet opens google doc , returns text in cell of table, follows:

function getitemstatus(docurl) {    var doc = documentapp.openbyurl(docurl);    var docbody = doc.getbody();    var doctbls = docbody.gettables();    var itmtbl = doctbls[0];    var itmstatus = itmtbl.getcell(3,1).gettext();    return itmstatus;   } 

when attempt run function in spreadsheet, returns error:

error: not have permission call openbyurl (line 39, file "code")

if create function call above function directly works fine , returns value after.

function testitemget() {    var docurl = ("...");    itmstr = getitemstatus(docurl);    logger.log("the item's value " + itmstr) } 

resulting log file:

[13-07-17 19:27:39:130 edt] starting execution

[13-07-17 19:27:39:387 edt] documentapp.openbyurl([...]) [0.253 seconds]

[13-07-17 19:27:39:388 edt] document.getbody() [0 seconds]

[13-07-17 19:27:39:388 edt] body.gettables() [0 seconds]

[13-07-17 19:27:39:388 edt] table.getcell([3, 1]) [0 seconds]

[13-07-17 19:27:39:389 edt] tablecell.gettext() [0 seconds]

[13-07-17 19:27:39:389 edt] logger.clear() [0 seconds]

[13-07-17 19:27:39:389 edt] logger.log([the item's value deferred , []]) [0 seconds]

[13-07-17 19:27:39:390 edt] execution succeeded [0.256 seconds total runtime]

from have googled, long have edit or owner rights document should able this.

i pretty new google apps scripting please gentle. :)

thank ahead of time , patience!

custom functions run limited permissions, , cannot take advantages of services rely on user credentials, including documentapp.

https://developers.google.com/apps-script/execution_custom_functions#permissions