MongoDB sharded cluster. Please use Mongo shell to reproduce this issue: mongos> use sharding_test; mongos> sh.enableSharding( "sharding_test" ); mongos> db.createCollection( 'test1' ); mongos> sh.shardCollection( "sharding_test.test1" , { 'a1234567890' : 1, 'b1234567890' : 1, 'c1234567890' : 1 }); { "collectionsharded" : "sharding_test.test1" , "ok" : 1 } mongos> db.test1.insert({ 'a1234567890' : 1, 'b1234567890' : 1, 'c1234567890' : 1 }); mongos> db.test1.getIndexes(); [ { "v" : 1, "key" : { "_id" : 1 }, "ns" : "sharding_test.test1" , "name" : "_id_" }, { "v" : 1, "key" : { "a1234567890" : 1, "b1234567890" : 1, "c1234567890" : 1 }, "ns" : "sharding_test.test1" , "name" : "a1234567890_1_b1234567890_1_c1234567890_1" } ] mongos> db.createCollection( 'test2' ); mongos> sh.shardCollection( "sharding_test.test2" , { 'a1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890' : 1, 'b1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890' : 1, 'c1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890' : 1 }); { "collectionsharded" : "sharding_test.test2" , "ok" : 1 } mongos> db.test2.insert({ "a1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890" : 1, "b1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890" : 1, "c1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890" : 1 }); mongos> db.test2.getIndexes(); [ { "v" : 1, "key" : { "_id" : 1 }, "ns" : "sharding_test.test2" , "name" : "_id_" } ] As you can see there is index for sharding key of sharding_test.test1 collection and no index for sharding key of sharding_test.test2. Moreover there is no any exception observed. Expected result (any one is acceptible): at least throw exception create index for sharding key if there are no suitable indexes which overlap sharding key introduce possibility manually specify index name for sharding key