CSV importer wizard breaks when custom field name contains a plus

Given a CSV file containing:

type,summary,customfield+
Bug, "Hooray", TestValue

If this is fed into the CSV mapping wizard, it breaks on step 2 of 5 (ConfigureCsvMapping.jspa) with a big uninformative error in the browser:

HTTP Status 404 - Could not execute action [ConfigureCsvMapping]:null<p><small><small><pre>webwork.action.ResultException at webwork.action.factory.ParametersActionFactoryProxy.getActionImpl(ParametersActionFactoryProxy.java:55) at webwork.action.factory.ChainingActionFactoryProxy.getActionImpl(ChainingActionFactoryProxy.java:52) at webwork.action.factory.DefaultActionFactory.getActionImpl(DefaultActionFactory.java:125) at webwork.action.factory.ActionFactory.getAction(ActionFactory.java:63) at webwork.dispatcher.GenericDispatcher.executeAction(GenericDispatcher.java:120) at com.atlassian.jira.web.dispatcher.JiraServletDispatcher.service(JiraServletDispatcher.java:209) at javax.servlet.http.HttpServlet.service(HttpServlet.java:802) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173) at com.atlassian.jira.web.filters.AccessLogFilter.doFilter(AccessLogFilter.java:73) at
...

and a more informative error in the logs:

2007-06-12 11:21:22,926 http-8080-Processor23 WARN [webwork.util.BeanUtil] Could not set parameter "field.customfield+" with value "[Ljava.lang.String;@14005e1" on class class com.atlassian.jira.web.action.util.csv.ConfigureCsvMapping:java.lang.ArrayIndexOutOfBoundsException: 0
java.lang.ArrayIndexOutOfBoundsException: 0
at webwork.util.Query.<init>(Query.java:67)
at webwork.util.Query.getQuery(Query.java:34)
at webwork.util.Query.<init>(Query.java:136)
at webwork.util.Query.getQuery(Query.java:34)
at webwork.util.BeanUtil.setProperty(BeanUtil.java:239)
at webwork.util.BeanUtil.setProperties(BeanUtil.java:182)
at webwork.action.factory.ParametersActionFactoryProxy.getActionImpl(ParametersActionFactoryProxy.java:53)
at webwork.action.factory.ChainingActionFactoryProxy.getActionImpl(ChainingActionFactoryProxy.java:52)
at webwork.action.factory.DefaultActionFactory.getActionImpl(DefaultActionFactory.java:125)
at webwork.action.factory.ActionFactory.getAction(ActionFactory.java:63)
at webwork.dispatcher.GenericDispatcher.executeAction(GenericDispatcher.java:120)
at com.atlassian.jira.web.dispatcher.JiraServletDispatcher.service(JiraServletDispatcher.java:209)

It seems we're not escaping something from Webwork.