[logback-user] NoClassDefFoundError for ThrowableProxy?

Why am I getting this warning in my log file? My WEB-INF/lib directory contains logback-classic-0.9.11.jar and that jar file contains ThrowableProxy.class. 2008-12-01 18:46:59 WARN ContainerBackgroundProcessor[StandardEngine[Catalina]] org.springframework.beans.factory.support.DisposableBeanAdapter.invokeCustomDestroyMethod:217, Invocation of destroy method 'close' failed on bean with name 'dataSource-db2': java.lang.NoClassDefFoundError: ch/qos/logback/classic/spi/ThrowableProxy