Did you update your copy of pg as well as webwork2? It looks to me like your version of the Value package might be out of date (or the versions of files within the library are not consistent). Try
cvs update -r rel-2-4-patches
within the pg directory to make sure that they are all at the current version. If that doesn't work, try
cvs update -A
instead to get the HEAD version. It might be that some changes have not been backported to the rel-2-4-patches release properly.