Received stack trace when displaying popup wiki formatting help from comments help link

Received servlet error when displaying popup wiki formatting help from comments help link. Page header displayed ok. Content displayed stack trace below:

An error occurred whilst rendering this message. Please contact the administrators, and inform them of this bug. Details: ------- java.lang.NullPointerException at org.apache.velocity.context.InternalContextAdapterImpl.get(InternalContextAdapterImpl.java:162) at org.apache.velocity.runtime.parser.node.ASTReference.getVariableValue(ASTReference.java:623) at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:151) at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:220) at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:230) at org.apache.velocity.Template.merge(Template.java:256) at org.apache.velocity.app.VelocityEngine.mergeTemplate(VelocityEngine.java:450) at org.apache.velocity.app.VelocityEngine.mergeTemplate(VelocityEngine.java:419) at com.atlassian.velocity.DefaultVelocityManager.getEncodedBody(DefaultVelocityManager.java:74) at com.atlassian.jira.plugin.webwork.JiraPluginWebworkVelocityServlet.service(JiraPluginWebworkVelocityServlet.java:50) at javax.servlet.http.HttpServlet.service(HttpServlet.java:802) at