Exception stack trace emailed when an Issue comment is edited

When our users edit comments on JIRA issues, the watchers are emailed with a Java exception stack trace. This happens consistently, every time a comment is edited. We are experiencing this on our project named "BUGS". As it may be relevant, note that we have the Zendesk integration enabled and we only know of this occurring with issues originating in Zendesk; we don't know that it occurs or not with issues originating in JIRA.

I've attached a screenshot of the stack trace and I'll include it here as text.

An error occurred whilst rendering this message. Please contact the administrators, and inform them of this bug. Details: ------- org.apache.velocity.exception.MethodInvocationException: Invocation of method 'diff' in class com.atlassian.jira.mail.DiffUtils threw exception java.lang.NullPointerException at getEncodedBodyFromContent[line 6, column 34] at org.apache.velocity.runtime.parser.node.ASTMethod.handleInvocationException(ASTMethod.java:337) at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:284) at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:262) at org.apache.velocity.runtime.parser.node.ASTReference.value(ASTReference.java:507) at org.apache.velocity.runtime.parser.node.ASTExpression.value(ASTExpression.java:71) at org.apache.velocity.runtime.parser.node.ASTSetDirective.render(ASTSetDirective.java:142) at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:336) at org.apache.velocity.runtime.RuntimeInstance.render(RuntimeInstance.java:1277) at org.apache.velocity.runtime.RuntimeInstance.evaluate(RuntimeInstance.java:1216) at org.apache.velocity.runtime.RuntimeInstance.evaluate(RuntimeInstance.java:1165) at org.apache.velocity.app.VelocityEngine.evaluate(VelocityEngine.java:219) at com.atlassian.velocity.DefaultVelocityManager.writeEncodedBodyForContent(DefaultVelocityManager.java:86) at com.atlassian.jira.template.velocity.DefaultVelocityTemplatingEngine$DefaultRenderRequest.toWriterImpl(DefaultVelocityTemplatingEngine.java:157) at com.atlassian.jira.template.velocity.DefaultVelocityTemplatingEngine$DefaultRenderRequest.asPlainText(DefaultVelocityTemplatingEngine.java:128) at com.atlassian.jira.template.velocity.DefaultVelocityTemplatingEngine$DefaultRenderRequest$1.with(DefaultVelocityTemplatingEngine.java:108) at com.atlassian.jira.template.velocity.DefaultVelocityTemplatingEngine$DefaultRenderRequest$StringRepresentation.toString(DefaultVelocityTemplatingEngine.java:88) at com.atlassian.jira.template.velocity.DefaultVelocityTemplatingEngine$DefaultRenderRequest.asPlainText(DefaultVelocityTemplatingEngine.java:104) at com.atlassian.jira.mail.MailingListCompiler$1.processRecipient(MailingListCompiler.java:307) at com.atlassian.jira.mail.NotificationRecipientProcessor.process(NotificationRecipientProcessor.java:39) at com.atlassian.jira.mail.MailingListCompiler.addMailsToQueue(MailingListCompiler.java:337) at com.atlassian.jira.mail.MailingListCompiler.access$400(MailingListCompiler.java:47) at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.addEmailsToQueue(MailingListCompiler.java:495) at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.sendLists(MailingListCompiler.java:465) at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.sendNoLevelsIgnoreGroup(MailingListCompiler.java:445) at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.sendForEvent(MailingListCompiler.java:434) at com.atlassian.jira.mail.MailingListCompiler.sendLists(MailingListCompiler.java:140) at com.atlassian.jira.mail.IssueMailQueueItem.send(IssueMailQueueItem.java:149) at com.atlassian.mail.queue.MailQueueImpl.sendBuffer(MailQueueImpl.java:66) at com.atlassian.jira.service.services.mail.MailQueueService.run(MailQueueService.java:28) at com.atlassian.jira.service.JiraServiceContainerImpl.run(JiraServiceContainerImpl.java:61) at com.atlassian.jira.service.ServiceRunner.execute(ServiceRunner.java:48) at org.quartz.core.JobRunShell.run(JobRunShell.java:195) at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:520) Caused by: java.lang.NullPointerException at java.util.regex.Matcher.getTextLength(Matcher.java:1140) at java.util.regex.Matcher.reset(Matcher.java:291) at java.util.regex.Matcher.<init>(Matcher.java:211) at java.util.regex.Pattern.matcher(Pattern.java:888) at com.atlassian.diff.WordLevelDiffer.tokenize(WordLevelDiffer.java:153) at com.atlassian.diff.WordLevelDiffer.diffLine(WordLevelDiffer.java:72) at com.atlassian.diff.DiffViewBean.createWordLevelDiff(DiffViewBean.java:123) at com.atlassian.jira.mail.DiffUtils.diff(DiffUtils.java:23) at sun.reflect.GeneratedMethodAccessor667.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.doInvoke(UberspectImpl.java:381) at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:370) at com.atlassian.velocity.htmlsafe.introspection.UnboxingMethod.invoke(UnboxingMethod.java:30) at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:270) ... 31 more