I'm converting my Hibernate persistence layer to use annotations, but when I deploy the app I get the following mysterious error:

Code:
org.hibernate.MappingException: Could not determine type for: java.util.Set, for columns: [org.hibernate.mapping.Column(users)]
	at org.hibernate.mapping.SimpleValue.getType(SimpleValue.java:266)
	at org.hibernate.mapping.SimpleValue.isValid(SimpleValue.java:253)
	at org.hibernate.mapping.Property.isValid(Property.java:185)
	at org.hibernate.mapping.PersistentClass.validate(PersistentClass.java:440)
	at org.hibernate.mapping.RootClass.validate(RootClass.java:192)
	at org.hibernate.cfg.Configuration.validate(Configuration.java:1102)
	at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1287)
	at org.hibernate.cfg.AnnotationConfiguration.buildSessionFactory(AnnotationConfiguration.java:915)
	at org.springframework.orm.hibernate3.LocalSessionFactoryBean.newSessionFactory(LocalSessionFactoryBean.java:825)
	at org.springframework.orm.hibernate3.LocalSessionFactoryBean.afterPropertiesSet(LocalSessionFactoryBean.java:751)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1091)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:396)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:233)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:145)
	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:186)
	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:106)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1046)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:857)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:378)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:233)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:145)
	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:186)
	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:106)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1046)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:857)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:378)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:233)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:145)
	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:186)
	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:106)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1046)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:857)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:378)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:233)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:145)
	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:186)
	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:106)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1046)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:857)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:378)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:233)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:145)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeansOfType(DefaultListableBeanFactory.java:211)
	at org.springframework.context.support.AbstractApplicationContext.getBeansOfType(AbstractApplicationContext.java:596)
	at org.springframework.context.support.AbstractApplicationContext.registerListeners(AbstractApplicationContext.java:478)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:310)
	at org.springframework.web.context.support.AbstractRefreshableWebApplicationContext.refresh(AbstractRefreshableWebApplicationContext.java:139)
	at org.springframework.web.context.ContextLoader.createWebApplicationContext(ContextLoader.java:246)
	at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:184)
	at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:49)
	at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:3729)
	at org.apache.catalina.core.StandardContext.start(StandardContext.java:4187)
	at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:759)
	at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:739)
	at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:524)
	at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:809)
	at org.apache.catalina.startup.HostConfig.deployWARs(HostConfig.java:698)
	at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:472)
	at org.apache.catalina.startup.HostConfig.start(HostConfig.java:1122)
	at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:310)
	at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:119)
	at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1021)
	at org.apache.catalina.core.StandardHost.start(StandardHost.java:718)
	at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1013)
	at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:442)
	at org.apache.catalina.core.StandardService.start(StandardService.java:450)
	at org.apache.catalina.core.StandardServer.start(StandardServer.java:709)
	at org.apache.catalina.startup.Catalina.start(Catalina.java:551)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:585)
	at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:294)
	at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:432)
The error doesn't tell me exactly where the problem is, but I gather that it's most likely associated with one of several many-to-many relationships to the User class. Here is one of those getUser() methods with the associated annotation:

Code:
	@ManyToMany(fetch=FetchType.EAGER)
    @JoinTable(name="reservation_user", joinColumns = { 
        @JoinColumn(name="reservation_key", nullable=false, updatable=false) }, inverseJoinColumns = { 
        @JoinColumn(name="user_key", nullable=false, updatable=false) })
    @Cascade( {CascadeType.SAVE_UPDATE} )
    public Set<User> getUsers() {
		if (users == null){
			users = new HashSet<User>();
		}
		return users;
	}
This annotation was generated by hbm2java. I'm very new to annotations, but can anyone more experienced see anything wrong? Am I looking in the right place?