error stack trace in email notification on issue update

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 templates/email/html/includes/patterns/worklog-top.vm[line 9, column 46]
 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.ASTBlock.render(ASTBlock.java:72)
 at org.apache.velocity.runtime.parser.node.ASTElseIfStatement.render(ASTElseIfStatement.java:92)
 at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:106)
 at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
 at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:87)
 at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
 at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:87)
 at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:336)
 at org.apache.velocity.runtime.directive.Parse.render(Parse.java:263)
 at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:175)
 at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
 at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:87)
 at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
 at org.apache.velocity.runtime.directive.VelocimacroProxy.render(VelocimacroProxy.java:212)
 at org.apache.velocity.runtime.directive.RuntimeMacro.render(RuntimeMacro.java:247)
 at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:175)
 at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:336)
 at org.apache.velocity.runtime.RuntimeInstance.render(RuntimeInstance.java:1276)
 at org.apache.velocity.runtime.RuntimeInstance.evaluate(RuntimeInstance.java:1215)
 at org.apache.velocity.runtime.RuntimeInstance.evaluate(RuntimeInstance.java:1164)
 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:110)
 at com.atlassian.jira.mail.builder.EmailRenderer.renderEmailBody(EmailRenderer.java:113)
 at com.atlassian.jira.mail.builder.EmailRenderer.render(EmailRenderer.java:166)
 at com.atlassian.jira.mail.builder.EmailBuilder.renderNow(EmailBuilder.java:162)
 at com.atlassian.jira.mail.builder.EmailBuilder.renderNowAsQueueItem(EmailBuilder.java:152)
 at com.atlassian.jira.mail.MailingListCompiler$1.processRecipient(MailingListCompiler.java:311)
 at com.atlassian.jira.mail.NotificationRecipientProcessor.process(NotificationRecipientProcessor.java:39)
 at com.atlassian.jira.mail.MailingListCompiler.addMailsToQueue(MailingListCompiler.java:332)
 at com.atlassian.jira.mail.MailingListCompiler.access$200(MailingListCompiler.java:47)
 at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.addEmailsToQueue(MailingListCompiler.java:455)
 at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.sendLists(MailingListCompiler.java:425)
 at com.atlassian.jira.mail.MailingListCompiler$NotificationCompiler.sendForEvent(MailingListCompiler.java:378)
 at com.atlassian.jira.mail.MailingListCompiler.sendLists(MailingListCompiler.java:120)
 at com.atlassian.jira.mail.IssueMailQueueItem.send(IssueMailQueueItem.java:150)
 at com.atlassian.mail.queue.MailQueueImpl.sendBuffer(MailQueueImpl.java:66)
 at com.atlassian.jira.mail.JiraMailQueue$1.apply(JiraMailQueue.java:57)
 at com.atlassian.jira.mail.JiraMailQueue$1.apply(JiraMailQueue.java:53)
 at com.atlassian.jira.util.velocity.DefaultVelocityRequestContextFactory.runWithStaticBaseUrl(DefaultVelocityRequestContextFactory.java:127)
 at com.atlassian.jira.util.DefaultBaseUrl.runWithStaticBaseUrl(DefaultBaseUrl.java:58)
 at com.atlassian.jira.mail.JiraMailQueue.sendBuffer(JiraMailQueue.java:52)
 at com.atlassian.jira.service.services.mail.MailQueueService.run(MailQueueService.java:30)
 at com.atlassian.jira.service.JiraServiceContainerImpl.run(JiraServiceContainerImpl.java:66)
 at com.atlassian.jira.service.ServiceRunner.runService(ServiceRunner.java:77)
 at com.atlassian.jira.service.ServiceRunner.runServiceId(ServiceRunner.java:54)
 at com.atlassian.jira.service.ServiceRunner.runJob(ServiceRunner.java:37)
 at com.atlassian.scheduler.core.JobLauncher.runJob(JobLauncher.java:153)
 at com.atlassian.scheduler.core.JobLauncher.launchAndBuildResponse(JobLauncher.java:118)
 at com.atlassian.scheduler.core.JobLauncher.launch(JobLauncher.java:97)
 at com.atlassian.scheduler.quartz1.Quartz1Job.execute(Quartz1Job.java:32)
 at org.quartz.core.JobRunShell.run(JobRunShell.java:223)
 at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:549) 
Caused by: java.lang.NullPointerException
 at java.util.regex.Matcher.getTextLength(Matcher.java:1283)
 at java.util.regex.Matcher.reset(Matcher.java:309)
 at java.util.regex.Matcher.<init>(Matcher.java:229)
 at java.util.regex.Pattern.matcher(Pattern.java:1093)
 at com.atlassian.diff.WordLevelDiffer.tokenize(WordLevelDiffer.java:176)
 at com.atlassian.diff.WordLevelDiffer.diffLine(WordLevelDiffer.java:95)
 at com.atlassian.diff.DiffViewBean.createWordLevelDiff(DiffViewBean.java:124)
 at com.atlassian.jira.mail.DiffUtils.diff(DiffUtils.java:23)
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
 at java.lang.reflect.Method.invoke(Method.java:483)
 at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.doInvoke(UberspectImpl.java:385)
 at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:374)
 at com.atlassian.velocity.htmlsafe.introspection.UnboxingMethod.invoke(UnboxingMethod.java:30)
 at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:270) ... 61 more