An error occured :(

Error code : err:XPTY0004
Error stack trace : Stopped at /srv/basex/webapp/synopsx/workspace/acadmed-biblio/models/index.xqm, 74/22: [XPTY0004] Item expected, empty sequence found. Stack Trace: - /srv/basex/webapp/synopsx/models/synopsx.xqm, 169/36 - /srv/basex/webapp/synopsx/models/synopsx.xqm, 169/51