An error occured :(

Error code : err:XPTY0004
Error stack trace : Stopped at /sites/ihrimweb/resource/basex_ancmod/webapp/synopsx/workspace/pam/models/tei.xqm, 245/58: [XPTY0004] Cannot convert empty-sequence() to xs:string: $indexId := (). Stack Trace: - /sites/ihrimweb/resource/basex_ancmod/webapp/synopsx/workspace/pam/models/tei.xqm, 193/30 - /sites/ihrimweb/resource/basex_ancmod/webapp/synopsx/models/synopsx.xqm, 169/36 - /sites/ihrimweb/resource/basex_ancmod/webapp/synopsx/models/synopsx.xqm, 169/51