When JIRA is sending a custom notification event upon transitioning a workflow transition, the following exceptions will be thrown in the logs file:

2012-12-05 16:47:38,005 http-bio-8082-exec-24 WARN admin 1007x663x1 81e19m 0:0:0:0:0:0:0:1 /secure/WorkflowUIDispatcher.jspa [jira.plugins.webhooks.WebHookRegistrationManager] Failed to publish web-hooks for event com.atlassian.jira.event.issue.IssueEvent
java.lang.NullPointerException: null value
	at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:204)
	at com.google.common.collect.ImmutableMap.entryOf(ImmutableMap.java:139)
	at com.google.common.collect.ImmutableMap$Builder.put(ImmutableMap.java:178)
	at com.atlassian.jira.plugins.webhooks.mapper.EventTypeInfoDecorator.toMap(EventTypeInfoDecorator.java:28)
	at com.atlassian.jira.plugins.webhooks.impl.JiraEventSerializerFactory.create(JiraEventSerializerFactory.java:52)
	at com.atlassian.jira.plugins.webhooks.impl.JiraEventSerializerFactory.create(JiraEventSerializerFactory.java:27)
	at com.atlassian.jira.plugins.webhooks.WebHookRegistration.getEventSerializer(WebHookRegistration.java:39)
	at com.atlassian.jira.plugins.webhooks.WebHookRegistrationManager.onEvent(WebHookRegistrationManager.java:136)  <+2> (DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
[.....]
Tested the same behavior on JIRA 5.1.7, and the same problem is not reproducible. 
Steps to reproduce this:
	
Create a new event in JIRA: Administration > Events > Add New Events
	
Configure JIRA workflow to use the event in one of the transition
	
Configure the notification scheme to send notifications to the intended recipient(s)
	
Configre a project to use the related workflow scheme and notification scheme
	
Create a test issue in JIRA, and transition the issue that is using the created custom event