ZK Studio problem

Servlet.service() for servlet zkLoader threw exception
org.zkoss.zk.ui.UiException: The class attribute is required, [SYS file:E:/apache-tomcat-6.0.14/webapps/Sinokru/new_file.zul line 4 col 79]
	at org.zkoss.zk.ui.metainfo.Parser.parse(Parser.java:249)
	at org.zkoss.zk.ui.metainfo.Parser.parse(Parser.java:205)
	at org.zkoss.zk.ui.metainfo.Parser.parse(Parser.java:101)
	at org.zkoss.zk.ui.metainfo.PageDefinitions$MyLoader.parse(PageDefinitions.java:196)
	at org.zkoss.web.util.resource.ResourceLoader.load(ResourceLoader.java:94)
	at org.zkoss.util.resource.ResourceCache$Info.load(ResourceCache.java:229)
	at org.zkoss.util.resource.ResourceCache$Info.<init>(ResourceCache.java:203)
	at org.zkoss.util.resource.ResourceCache.get(ResourceCache.java:144)
	at org.zkoss.web.util.resource.ResourceCaches.get(ResourceCaches.java:131)
	at org.zkoss.zk.ui.metainfo.PageDefinitions.getPageDefinition(PageDefinitions.java:142)
	at org.zkoss.zk.ui.impl.AbstractUiFactory.getPageDefinition(AbstractUiFactory.java:115)
	at org.zkoss.zk.ui.http.DHtmlLayoutServlet.process(DHtmlLayoutServlet.java:219)
	at org.zkoss.zk.ui.http.DHtmlLayoutServlet.doGet(DHtmlLayoutServlet.java:167)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:263)
	at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
	at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:584)
	at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
	at java.lang.Thread.run(Unknown Source)