In mongo shell run: db.runCommand({aggregate: "test" , pipeline: {}}) mongod will report a stack trace of the triggered massert: 2016-08-12T15:20:31.243-0400 I - [conn1] Assertion: 13111:wrong type for field (pipeline) 3 != 4 src/mongo/bson/bsonelement.h 657 2016-08-12T15:20:31.259-0400 I CONTROL [conn1] 0x10f6121aa 0x10f5b92f1 0x10f5a6531 0x10f5a6228 0x10ec19b23 0x10ec320a4 0x10f0a3f97 0x10ed9adb9 0x10ed99e84 0x10f28edac 0x10ef28fdd 0x10ec1c6c5 0x10f5984a1 0x10f598b11 0x7fff8ae3f99d 0x7fff8ae3f91a 0x7fff8ae3d351 ----- BEGIN BACKTRACE ----- { "backtrace" :[{ "b" : "10EC0E000" , "o" : "A041AA" , "s" : "_ZN5mongo15printStackTraceERNSt3__113basic_ostreamIcNS0_11char_traitsIcEEEE" },{ "b" : "10EC0E000" , "o" : "9AB2F1" , "s" : "_ZN5mongo10logContextEPKc" },{ "b" : "10EC0E000" , "o" : "998531" , "s" : "_ZN5mongo23msgassertedWithLocationEiPKcS1_j" },{ "b" : "10EC0E000" , "o" : "998228" , "s" : 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, "buildId" : "146E97FEDC803179A22768A89B5AC66E" }, { "path" : "/usr/lib/libSystem.B.dylib" , "machType" : 6, "b" : "7FFF8F1D2000" , "vmaddr" : "7FFF8C84F000" , "buildId" : "ECAA07BEA1143461AA13C57838775FC2" }, { "path" : "/usr/lib/libc++.1.dylib" , "machType" : 6, "b" : "7FFF962C1000" , "vmaddr" : "7FFF9393E000" , "buildId" : "8FC3D139805534989AC56467CB7F4D14" }, { "path" : "/usr/lib/system/libcache.dylib" , "machType" : 6, "b" : "7FFF8612D000" , "vmaddr" : "7FFF837AA000" , "buildId" : "9548AAE92AB735259ECEA2A7C4688447" }, { "path" : "/usr/lib/system/libcommonCrypto.dylib" , "machType" : 6, "b" : "7FFF87263000" , "vmaddr" : "7FFF848E0000" , "buildId" : "9373226134B43914B7A290A81A182DBA" }, { "path" : "/usr/lib/system/libcompiler_rt.dylib" , "machType" : 6, "b" : "7FFF88B23000" , "vmaddr" : "7FFF861A0000" , "buildId" : "A13ECF69F59F38AE86097B731450FBCD" }, { "path" : "/usr/lib/system/libcopyfile.dylib" , "machType" : 6, "b" : "7FFF94EBF000" , "vmaddr" : "7FFF9253C000" , "buildId" : 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{ "path" : "/usr/lib/system/libxpc.dylib" , "machType" : 6, "b" : "7FFF8487A000" , "vmaddr" : "7FFF81EF7000" , "buildId" : "54D1328E054E3DAA89E2375722F9D18F" }, { "path" : "/usr/lib/libobjc.A.dylib" , "machType" : 6, "b" : "7FFF95122000" , "vmaddr" : "7FFF9279F000" , "buildId" : "7489D2D61EFD3414B18D2AECCCC90286" }, { "path" : "/usr/lib/libauto.dylib" , "machType" : 6, "b" : "7FFF8CB47000" , "vmaddr" : "7FFF8A1C4000" , "buildId" : "999E610F41FC32A3ADCA5EC049B65DFB" }, { "path" : "/usr/lib/libc++abi.dylib" , "machType" : 6, "b" : "7FFF873C4000" , "vmaddr" : "7FFF84A41000" , "buildId" : "922EFB360E9E315B8270E81AC43472C0" }, { "path" : "/usr/lib/libDiagnosticMessagesClient.dylib" , "machType" : 6, "b" : "7FFF8726F000" , "vmaddr" : "7FFF848EC000" , "buildId" : "4243B6B421E9355B9C5A95A216233B96" } ] }} mongod(_ZN5mongo15printStackTraceERNSt3__113basic_ostreamIcNS0_11char_traitsIcEEEE+0x3A) [0x10f6121aa] mongod(_ZN5mongo10logContextEPKc+0x171) [0x10f5b92f1] mongod(_ZN5mongo23msgassertedWithLocationEiPKcS1_j+0x301) [0x10f5a6531] mongod(_ZN5mongo23msgassertedWithLocationEiRKNSt3__112basic_stringIcNS0_11char_traitsIcEENS0_9allocatorIcEEEEPKcj+0x18) [0x10f5a6228] mongod(_ZNK5mongo11BSONElement3chkEi+0x2F3) [0x10ec19b23] mongod(_ZNK5mongo11BSONElement5ArrayEv+0x24) [0x10ec320a4] mongod(_ZN5mongo8Pipeline23aggSupportsWriteConcernERKNS_7BSONObjE+0x57) [0x10f0a3f97] mongod(_ZN5mongo7Command3runEPNS_16OperationContextERKNS_3rpc16RequestInterfaceEPNS3_21ReplyBuilderInterfaceE+0x469) [0x10ed9adb9] mongod(_ZN5mongo7Command11execCommandEPNS_16OperationContextEPS0_RKNS_3rpc16RequestInterfaceEPNS4_21ReplyBuilderInterfaceE+0x964) [0x10ed99e84] mongod(_ZN5mongo11runCommandsEPNS_16OperationContextERKNS_3rpc16RequestInterfaceEPNS2_21ReplyBuilderInterfaceE+0x23C) [0x10f28edac] mongod(_ZN5mongo16assembleResponseEPNS_16OperationContextERNS_7MessageERNS_10DbResponseERKNS_11HostAndPortE+0xD3D) [0x10ef28fdd] mongod(_ZN5mongo23ServiceEntryPointMongod12_sessionLoopEPNS_9transport7SessionE+0x145) [0x10ec1c6c5] mongod(_ZN5mongo12_GLOBAL__N_17runFuncEPv+0x2D1) [0x10f5984a1] mongod(_ZNSt3__114__thread_proxyINS_5tupleIJNS_6__bindIRFPvS3_EJPN5mongo12_GLOBAL__N_17ContextEEEEEEEEES3_S3_+0x61) [0x10f598b11] libsystem_pthread.dylib(_pthread_body+0x83) [0x7fff8ae3f99d] libsystem_pthread.dylib(_pthread_body+0x0) [0x7fff8ae3f91a] libsystem_pthread.dylib(thread_start+0xD) [0x7fff8ae3d351] ----- END BACKTRACE -----