why some reference lable is null generated by tag -r

[zhugw]

After training

crfsuite learn -m review.model train.data

I got a model then I test it's effect

crfsuite tag -r -m review.model test.data > result

but I found many null

cat -n result | grep 'null'
    33	(null)	N
    35	(null)	N
    37	(null)	N
    46	(null)	N
    50	(null)	N
    59	(null)	N
    68	(null)	N
    77	(null)	N
    97	(null)	N
   118	(null)	N
   134	(null)	N
   139	(null)	N
   196	(null)	N
   205	(null)	N
   263	(null)	N
   273	(null)	N
   287	(null)	N
   288	(null)	N
   324	(null)	N
   330	(null)	N
   342	(null)	N
   343	(null)	N
   347	(null)	N
   349	(null)	N
   366	(null)	N

but actually in test.data they have tagged,

cat -n result | grep 'null' | awk '{print $1}' | xargs -i{} sed -n '{}p' test.data | awk '{print $1}'
SEV-KY-95
LJ-119
LJ-119
SEV-PY-50
CPU-CM-146
HX-116
LOG-HGL-137
HX-116
HH-94
TH-18
LOG-HK-92
CAM-HX-116
KY-95
GY-149
NET-HX-151
SYS-BL-30
NET-WY-97
NET-WY-97
NET-BK-100
SCN-KY-95
BAT-BNY-168
BAT-RY-53
KY-95
BAT-HX-151
KY-95