See linked issue for more details.

Full exception looks like:
Fri Jul 1 23:30:08 [Balancer] Assertion: 10181:not sharded:frequency.Post
0x5204fa 0x5ed3e9 0x686652 0x68ce86 0x50454b 0x505e04 0x6a50a0 0x7f46f8cb9a04 0x7f46f8277d4d
./mongos(_ZN5mongo11msgassertedEiPKc+0x12a) [0x5204fa]
./mongos(_ZN5mongo8DBConfig15getChunkManagerERKSsb+0x119) [0x5ed3e9]
./mongos(_ZN5mongo8Balancer11_moveChunksEPKSt6vectorIN5boost10shared_ptrINS_14BalancerPolicy9ChunkInfoEEESaIS6_EE+0xe2) [0x686652]
./mongos(_ZN5mongo8Balancer3runEv+0x5f6) [0x68ce86]
./mongos(_ZN5mongo13BackgroundJob7jobBodyEN5boost10shared_ptrINS0_9JobStatusEEE+0x12b) [0x50454b]
./mongos(_ZN5boost6detail11thread_dataINS_3_bi6bind_tIvNS_4_mfi3mf1IvN5mongo13BackgroundJobENS_10shared_ptrINS7_9JobStatusEEEEENS2_5list2INS2_5valueIPS7_EENSD_ISA_EEEEEEE3runEv+0x74) [0x505e04]
./mongos(thread_proxy+0x80) [0x6a50a0]
/lib/libpthread.so.0 [0x7f46f8cb9a04]
/lib/libc.so.6(clone+0x6d) [0x7f46f8277d4d]
Fri Jul 1 23:30:08 [Balancer] ~ScopedDbConnection: _conn != null
Fri Jul 1 23:30:08 [Balancer] caught exception while doing balance: not sharded:frequency.Post