error triggered when monster dies to spell

[seiyria]

Error: Invalid entity removal attempt. fire ant tried to remove null
    at exports.default (logger.js:7)
    at World.removeEntity (world.js:244)
    at Monster.removeSelf (character.js:353)
    at Monster.removeSelf (monster.js:9)
    at Monster.die (character.js:349)
    at Monster.takeDamage (character.js:333)
    at Force.hit (attack.js:190)
    at attack.js:145
    at finalize (attack.js:117)
    at attack.js:127exports.default @ logger.js:7removeEntity @ world.js:244removeSelf @ character.js:353removeSelf @ monster.js:9die @ character.js:349takeDamage @ character.js:333hit @ attack.js:190(anonymous function) @ attack.js:145finalize @ attack.js:117(anonymous function) @ attack.js:127
Console.js:221 [ World  ]   Invalid entity removal attempt. fire ant tried to remove null