panic: reflect: call of reflect.Value.Type on zero Value

goroutine 16 [running]:
runtime.panic(0x7c6a80, 0xc2080e7900)
/usr/local/go/src/pkg/runtime/panic.c:279 +0xf5
reflect.Value.Type(0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
/usr/local/go/src/pkg/reflect/value.go:1837 +0x92
github.com/mongodb/mongo-tools/mongoexport.extractFieldByName(0x7ffc90a36754, 0x11, 0x781020, 0xc2080c6270, 0x0, 0x0, 0x0, 0x0)
/data/mci/src/src/mongo-tools-repo/.gopath/src/github.com/mongodb/mongo-tools/mongoexport/csv.go:96 +0x198
github.com/mongodb/mongo-tools/mongoexport.(*CSVExportOutput).ExportDocument(0xc20809ae10, 0xc2080c6270, 0x0, 0x0)
/data/mci/src/src/mongo-tools-repo/.gopath/src/github.com/mongodb/mongo-tools/mongoexport/csv.go:65 +0x1b8
github.com/mongodb/mongo-tools/mongoexport.(*MongoExport).exportInternal(0xc208058000, 0x7f82d34ac4b8, 0xc2080ae020, 0x0, 0x0, 0x0)
/data/mci/src/src/mongo-tools-repo/.gopath/src/github.com/mongodb/mongo-tools/mongoexport/mongoexport.go:258 +0x3c6
github.com/mongodb/mongo-tools/mongoexport.(*MongoExport).Export(0xc208058000, 0x7f82d34ac4b8, 0xc2080ae020, 0x7f82d34ac4b8, 0x0, 0x0)
/data/mci/src/src/mongo-tools-repo/.gopath/src/github.com/mongodb/mongo-tools/mongoexport/mongoexport.go:281 +0x4d
main.main()
/data/mci/src/src/mongo-tools-repo/mongoexport/main/mongoexport.go:84 +0x96f

goroutine 19 [finalizer wait]:
runtime.park(0x41bb40, 0xcb5870, 0xca24e9)
/usr/local/go/src/pkg/runtime/proc.c:1369 +0x89
runtime.parkunlock(0xcb5870, 0xca24e9)
/usr/local/go/src/pkg/runtime/proc.c:1385 +0x3b
runfinq()
/usr/local/go/src/pkg/runtime/mgc0.c:2644 +0xcf
runtime.goexit()
/usr/local/go/src/pkg/runtime/proc.c:1445

goroutine 20 [syscall]:
os/signal.loop()
/usr/local/go/src/pkg/os/signal/signal_unix.go:21 +0x1e
created by os/signal.init·1
/usr/local/go/src/pkg/os/signal/signal_unix.go:27 +0x32

goroutine 26 [sleep]:
time.Sleep(0x1dcd6500)
/usr/local/go/src/pkg/runtime/time.goc:39 +0x31
gopkg.in/mgo%2ev2.(*mongoCluster).syncServersLoop(0xc208082000)
/data/mci/src/src/mongo-tools-repo/vendor/src/gopkg.in/mgo.v2/cluster.go:365 +0x249
created by gopkg.in/mgo%2ev2.newCluster
/data/mci/src/src/mongo-tools-repo/vendor/src/gopkg.in/mgo.v2/cluster.go:76 +0x171

goroutine 25 [chan receive]:
github.com/mongodb/mongo-tools/common/signals.Handle()
/data/mci/src/src/mongo-tools-repo/.gopath/src/github.com/mongodb/mongo-tools/common/signals/signals_unix.go:16 +0x163
created by main.main
/data/mci/src/src/mongo-tools-repo/mongoexport/main/mongoexport.go:15 +0x31

goroutine 17 [syscall]:
runtime.goexit()
/usr/local/go/src/pkg/runtime/proc.c:1445

goroutine 27 [IO wait]:
net.runtime_pollWait(0x7f82d34adb50, 0x72, 0x0)
/usr/local/go/src/pkg/runtime/netpoll.goc:146 +0x66
net.(*pollDesc).Wait(0xc20802c760, 0x72, 0x0, 0x0)
/usr/local/go/src/pkg/net/fd_poll_runtime.go:84 +0x46
net.(*pollDesc).WaitRead(0xc20802c760, 0x0, 0x0)
/usr/local/go/src/pkg/net/fd_poll_runtime.go:89 +0x42
net.(*netFD).Read(0xc20802c700, 0xc208025a70, 0x24, 0x24, 0x0, 0x7f82d34ac3c8, 0xb)
/usr/local/go/src/pkg/net/fd_unix.go:242 +0x34c
net.(*conn).Read(0xc2080ae038, 0xc208025a70, 0x24, 0x24, 0x0, 0x0, 0x0)
/usr/local/go/src/pkg/net/net.go:122 +0xe7
gopkg.in/mgo%2ev2.fill(0x7f82d34adc58, 0xc2080ae038, 0xc208025a70, 0x24, 0x24, 0x0, 0x0)
/data/mci/src/src/mongo-tools-repo/vendor/src/gopkg.in/mgo.v2/socket.go:502 +0x7f
gopkg.in/mgo%2ev2.(*mongoSocket).readLoop(0xc208060540)
/data/mci/src/src/mongo-tools-repo/vendor/src/gopkg.in/mgo.v2/socket.go:519 +0x11f
created by gopkg.in/mgo%2ev2.newSocket
/data/mci/src/src/mongo-tools-repo/vendor/src/gopkg.in/mgo.v2/socket.go:166 +0x2ee

goroutine 28 [sleep]:
time.Sleep(0x12a05f200)
/usr/local/go/src/pkg/runtime/time.goc:39 +0x31
gopkg.in/mgo%2ev2.(*mongoServer).pinger(0xc208060460, 0xc208060401)
/data/mci/src/src/mongo-tools-repo/vendor/src/gopkg.in/mgo.v2/server.go:297 +0x11e
created by gopkg.in/mgo%2ev2.newServer
/data/mci/src/src/mongo-tools-repo/vendor/src/gopkg.in/mgo.v2/server.go:90 +0xf1