Fatal error initializing 'com.intellij.codeInsight.daemon.impl.DaemonListeners'

I'm creating a custom language plugin that compiles and prepares fine. But when I activate the plugin, IDEA shows this error message when trying to open a project:

Fatal error initializing 'com.intellij.codeInsight.daemon.impl.DaemonListeners'

What could be the reason for this? Again, there are no compile errors in my plugin JAR.