* merged from main

apt-pkg/algorithms.cc

@@ -512,7 +512,7 @@ void pkgProblemResolver::MakeScores()
          on the same level. We enhance the score of installed packages 
 	 if those are not obsolete
       */
-      if (I->CurrentVer != 0 && Cache[I].CandidateVerIter(Cache).Downloadable())
+      if (I->CurrentVer != 0 && Cache[I].CandidateVer != 0 && Cache[I].CandidateVerIter(Cache).Downloadable())
 	 Score += 1;
    }