Getting error on merge

3000: System error: 3000: Graph[1733], indexwrite[1746]: SLAVE 10.144.72.31:16600: assert(SELF::isItem(pos)) failed - file: /var/jenkins/workspace/LN-Candidate-3.10.2/LN/centos_5_x86_64/HPCC-Platform/thorlcr/msort/./../../system/jlib/jarray.tpp, line 129

An example workunit is:
W20130319-070145

We are failing on a merge of 10 files into a single file. We have not had problems with this sort of operation on a 702 build.