HANGUPCAUSE_KEYS() always returns an empty string

anyone has any idea why? i pretty much try to run the example on the wiki https://wiki.asterisk.org/wiki/display/AST/Hangup+Cause, and also tried calling the function from the h extension, but it always returns nothing. am i supposed to activate something? thanks.

edit: asterisk version is 11.5

i still found no way to make this work, not even on asterisk 13. i’m just using the same dialplan as here wiki.asterisk.org/wiki/display/AST/Hangup+Cause