empty date headers throw IllegalArgumentException

We have a production environment in which the Jetty 6.1.9 server is receiving requests for static content with a bogus request header: If-Modified-Since with no date. An IllegalArgumentException ensues:

[25 17:10:12.239 pool-1-thread-4 - /ima] 2008-06-25 17:10:12.615::WARN: EXCEPTION
[25 17:10:12.239 pool-1-thread-4 - /ima] EXCEPTION java.lang.IllegalArgumentException
java.lang.IllegalArgumentException: Cannot convert date:
at org.mortbay.jetty.HttpFields.getDateField(HttpFields.java:831)
at org.mortbay.jetty.Request.getDateHeader(Request.java:558)
at org.mortbay.jetty.servlet.DefaultServlet.passConditionalHeaders(DefaultServlet.java:559)
at org.mortbay.jetty.servlet.DefaultServlet.doGet(DefaultServlet.java:405)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1093)
at net.consilient.c4.console.servlets.filters.DisableCaching.doFilter(DisableCaching.java:49)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
at net.consilient.c4.console.servlets.filters.CharEncoding.doFilter(CharEncoding.java:58)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:360)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:726)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:320)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:505)
at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:828)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:514)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:380)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:395)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:885)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
at java.lang.Thread.run(Thread.java:619)

which is reasonable, however the response is 200 OK with empty content. If it's just a warning, then shouldn't the exception be caught lower down in passConditionalHeaders so request processing continues normally? Or if the content isn't to be served, then shouldn't the response be 5xx?

Thanks!