Error in 4.2 SP6 Upgrade Install

I'm upgrading a client from 4.2 SP5 to 4.2 SP6.  During the upgrade install, I get an error message that it is unable to upgrade the IS repository.  The error says to run ISRepositoryUtility to complete the upgrade.

I run ISRepositoryUtility using the upgrade instructions from the IS Admin Guide.  It starts running and then I get the following error:

Exception in thread "main" java.lang.UnsupportedClassVersionError: MITI/bridges/bo/mm/DataModelResource : Unsupported major.minor version 51.0

Can anyone help me resolve this issue?

Thanks!

-Dell