I run sumo in RHEL 6.X, sometimes sumo succeeds, sometimes failed as below:

$ cat server.log | sumo search "ERROR" | sumo search "Exception" | sumo parse "*:* *:* * [" as filename, date, hour, time, loginfo | sumo count hour date | render-basic

======
_Id   _count   date          hour
0     23       "2017-01-06   09
1     23       "2017-01-06   10
2     27       "2017-01-06   11
3     23       "2017-01-06   12
4     11       "2017-01-06   13
5     15       "2017-01-06   14
6     20       "2017-01-06   15
7     23       "2017-01-06   16
8     36       "2017-01-06   17
9     10       "2017-01-06   18
10    8        "2017-01-06   19
11    1        "2017-01-06   20
12    1        "2017-01-06   22
13    1        "2017-01-06   23
14    1        "2017-01-07   00
15    1        "2017-01-07   02
16    2        "2017-01-07   07
17    1        "2017-01-07   08
18    2        "2017-01-07   11
19    1        "2017-01-07   17
20    1        "2017-01-08   00
21    1        "2017-01-08   02
panic: assignment to entry in nil map

goroutine 6 [running]:
github.com/SumoLogic/sumoshell/group.Merger.Flush(0xc820010bd0, 0xc820018500, 0xc8200184f0)
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/group/Grouper.go:114 +0x202
github.com/SumoLogic/sumoshell/group.flush(0xc820010bd0, 0xc820018500, 0xc8200184f0, 0xc82000a540)
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/group/Grouper.go:125 +0x69
created by github.com/SumoLogic/sumoshell/group.NewMerger
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/group/Grouper.go:78 +0x151

goroutine 1 [syscall]:
syscall.Syscall(0x0, 0x0, 0xc82002b000, 0x1000, 0xc820184090, 0x4, 0x5a4450)
        /usr/local/go/src/syscall/asm_linux_amd64.s:18 +0x5
syscall.read(0x0, 0xc82002b000, 0x1000, 0x1000, 0xc820184090, 0x0, 0x0)
        /usr/local/go/src/syscall/zsyscall_linux_amd64.go:783 +0x5f
syscall.Read(0x0, 0xc82002b000, 0x1000, 0x1000, 0xc8201608e0, 0x0, 0x0)
        /usr/local/go/src/syscall/syscall_unix.go:160 +0x4d
os.(*File).read(0xc82003c008, 0xc82002b000, 0x1000, 0x1000, 0xe, 0x0, 0x0)
        /usr/local/go/src/os/file_unix.go:211 +0x53
os.(*File).Read(0xc82003c008, 0xc82002b000, 0x1000, 0x1000, 0xc820160900, 0x0, 0x0)
        /usr/local/go/src/os/file.go:95 +0x8a
bufio.(*Reader).fill(0xc82004fc68)
        /usr/local/go/src/bufio/bufio.go:97 +0x1e9
bufio.(*Reader).ReadSlice(0xc82004fc68, 0xc82001850a, 0x0, 0x0, 0x0, 0x0, 0x0)
        /usr/local/go/src/bufio/bufio.go:328 +0x21a
bufio.(*Reader).ReadLine(0xc82004fc68, 0x0, 0x0, 0x0, 0xc820184000, 0x0, 0x0)
        /usr/local/go/src/bufio/bufio.go:357 +0x53
github.com/SumoLogic/sumoshell/util.ConnectToReader(0x7f8ae6cc4218, 0xc820016500, 0x7f8ae6cc4290, 0xc82003c008)
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/util/Raw.go:64 +0x421
github.com/SumoLogic/sumoshell/util.ConnectToStdIn(0x7f8ae6cc4218, 0xc820016500)
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/util/Raw.go:36 +0xa3
main.connectAggOperator(0x7ffd71a95508, 0x5, 0xc82000a510, 0x3, 0x3, 0x8c8feb670ee1a900)
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/sumo/Main.go:70 +0x237
main.main()
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/sumo/Main.go:45 +0x226

goroutine 7 [semacquire]:
sync.runtime_Semacquire(0xc8200184f4)
        /usr/local/go/src/runtime/sema.go:43 +0x26
sync.(*Mutex).Lock(0xc8200184f0)
        /usr/local/go/src/sync/mutex.go:82 +0x1c4
github.com/SumoLogic/sumoshell/group.Merger.Process(0xc820010bd0, 0xc820018500, 0xc8200184f0, 0xc820155290)
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/group/Grouper.go:98 +0x30
github.com/SumoLogic/sumoshell/group.Merger.Write(0xc820010bd0, 0xc820018500, 0xc8200184f0, 0xc820155290)
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/group/Grouper.go:106 +0x3f
github.com/SumoLogic/sumoshell/group.(Merger).Write-fm(0xc820155290)
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/average/Average.go:31 +0x3f
github.com/SumoLogic/sumoshell/count.count.Flush(0xc820349360, 0xc8200b3c20, 0xc82025f760)
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/count/Count.go:38 +0x1c5
github.com/SumoLogic/sumoshell/count.(*count).Flush(0xc82025f780)
        :1 +0xa1
github.com/SumoLogic/sumoshell/group.Grouper.Flush(0xc82000a5c0, 0xc820010c00, 0xc820010bd0, 0xc820018500, 0xc8200184f0, 0xc82000a520, 0x2, 0x2, 0x0, 0x0)
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/group/Grouper.go:35 +0x99
github.com/SumoLogic/sumoshell/group.(*Grouper).Flush(0xc820016500)
        :4 +0x96
main.flush(0x7f8ae6cc41e8, 0xc820016500, 0xc82000a600)
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/sumo/Main.go:98 +0x5b
created by main.connectAggOperator
        /home/russell/code/go/src/github.com/SumoLogic/sumoshell/sumo/Main.go:69 +0x1f6

Please help fix it. Thank you very much!
Cheers,
Khon