ExecSQL NullPtrEx...conn timeout?

i am seeing NullPtrExs with the following stack trace:

java.lang.NullPointerException
at org.postgresql.Connection.ExecSQL(Connection.java:312)
at org.postgresql.jdbc2.Connection.commit(Connection.java:173)
at
com.commnav.sbh.framework.persist.JDBCConnection.disconnect(JDBCConnection.j
ava:416)
at
com.commnav.sbh.framework.persist.ConnectionPoolImpl.expire(ConnectionPoolIm
pl.java:245)
at com.commnav.sbh.framework.ObjectPool.clean(ObjectPool.java:178)
at com.commnav.sbh.framework.ObjectPool.run(ObjectPool.java:330)
at java.lang.Thread.run(Thread.java:484)

Our object pool disconnects unused open connections every few minutes.  Do
the connections themselves timeout?  is this a parameter somewhere?  I am
wondering if I am trying to close a connection that is already closed?

please respond directly to me - i'm not in the jdbc group.

thanks
chris