This gets written to the mongos log (and generates an exception in pymongo) on queries that look benign:

Wed Dec 28 09:39:15 [conn236] Assertion failure shards.size() s/commands_public.cpp 453
0x52b776 0x53662b 0x79a1f8 0x793175 0x76cf2b 0x7b4927 0x7c6cf1 0x5e6a07 0x3dbc80673d 0x3dbbcd44bd
mongos(_ZN5mongo12sayDbContextEPKc+0x96) [0x52b776]
mongos(_ZN5mongo8assertedEPKcS1_j+0xfb) [0x53662b]
mongos(_ZN5mongo15dbgrid_pub_cmds8CountCmd3runERKSsRNS_7BSONObjEiRSsRNS_14BSONObjBuilderEb+0x2108) [0x79a1f8]
mongos(_ZN5mongo7Command20runAgainstRegisteredEPKcRNS_7BSONObjERNS_14BSONObjBuilderEi+0x8b5) [0x793175]
mongos(_ZN5mongo14SingleStrategy7queryOpERNS_7RequestE+0x5cb) [0x76cf2b]
mongos(_ZN5mongo7Request7processEi+0x187) [0x7b4927]
mongos(_ZN5mongo21ShardedMessageHandler7processERNS_7MessageEPNS_21AbstractMessagingPortEPNS_9LastErrorE+0x71) [0x7c6cf1]
mongos(_ZN5mongo3pms9threadRunEPNS_13MessagingPortE+0x287) [0x5e6a07]
/lib64/libpthread.so.0 [0x3dbc80673d]
/lib64/libc.so.6(clone+0x6d) [0x3dbbcd44bd]