Setting sessionMode to native kills any SessionManager already configured

I just got done working through an issue trying to get single sign-on working with EhCacheManager, following some examples I found on the mailing list (except I was using Spring configuration). It wasn't working for me at first with this:

<bean id="securityManager" 
  class="org.apache.shiro.web.mgt.DefaultWebSecurityManager"> 
     <property name="realm" ref="factorlabRealm" /> 
     <property name="subjectFactory" ref="factorlabSubjectFactory" /> 
     <property name="cacheManager" ref="ssoCacheManager" /> 
     <property name="sessionManager" ref="sessionManager" /> 
     <property name="sessionMode" value="native" /> 
</bean> 

After lots of debugging, I got it to work with this:

<bean id="securityManager"
class="org.apache.shiro.web.mgt.DefaultWebSecurityManager">
<property name="sessionMode" value="native" />
<property name="realm" ref="factorlabRealm" />
<property name="subjectFactory" ref="factorlabSubjectFactory" />
<property name="cacheManager" ref="ssoCacheManager" />
<property name="sessionManager" ref="sessionManager" />
</bean>

The difference is moving the sessionMode to earlier in the property list, because the setter for sessionMode was replacing my configured
sessionManager with a new DefaultWebSessionManager.

This seems pretty fragile.