Error: (0) Undefined index: ack[BUG]

[syrinebyk1]

I recently encountered this error in one of our maps, Can someone explain the reason for this problem please?
URL: /nagvis/server/core/ajax_handler.php?mod=Map&act=getMapObjects&show=******&header_menu=1&hover_menu=1&context_menu=1&zoom=100&_ajaxid=1699350308
File: /usr/share/nagvis/share/server/core/classes/objects/NagVisObject.php
Line: 517
#0 /usr/share/nagvis/share/server/core/classes/objects/NagVisObject.php(517): nagvisExceptionErrorHandler(8, 'Undefined index...', '/usr/share/nagv...', 517, Array)
#1 /usr/share/nagvis/share/server/core/classes/objects/NagVisObject.php(498): NagVisObject::sortStatesByStateValues(2, 'ack', 0, 'ack', 'asc')
#2 [internal function]: NagVisObject::sortObjectsByState(Object(NagVisService), Object(NagVisService))
#3 /usr/share/nagvis/share/server/core/classes/objects/NagVisObject.php(328): usort(Array, Array)
#4 /usr/share/nagvis/share/server/core/classes/objects/NagVisObject.php(299): NagVisObject->getSortedObjectMembers()
#5 /usr/share/nagvis/share/server/core/classes/objects/NagVisObject.php(386): NagVisObject->getObjectInformation()
#6 /usr/share/nagvis/share/server/core/classes/objects/NagVisStatefulObject.php(425): NagVisObject->parseJson()
#7 /usr/share/nagvis/share/server/core/classes/NagVisMap.php(106): NagVisStatefulObject->parseJson()
#8 /usr/share/nagvis/share/server/core/classes/CoreModMap.php(325): NagVisMap->parseObjectsJson()
#9 /usr/share/nagvis/share/server/core/classes/CoreModMap.php(105): CoreModMap->getMapObjects()
#10 /usr/share/nagvis/share/server/core/functions/index.php(120): CoreModMap->handleAction()
#11 /usr/share/nagvis/share/server/core/ajax_handler.php(59): require('/usr/share/nagv...')
#12 {main}