diff --git a/jieba/posseg/viterbi.py b/jieba/posseg/viterbi.py
index 4451dcb..a95921c 100644
--- a/jieba/posseg/viterbi.py
+++ b/jieba/posseg/viterbi.py
@@ -1,5 +1,6 @@
 import operator
 MIN_FLOAT=-3.14e100
+MIN_INF=float("-inf")
 
 def get_top_states(t_state_v,K=4):
     items = t_state_v.items()
@@ -27,7 +28,7 @@ def viterbi(obs, states, start_p, trans_p, emit_p):
         if len(obs_states)==0: obs_states = all_states
 
         for y in obs_states:
-            (prob,state ) = max([(V[t-1][y0] + trans_p[y0].get(y,MIN_FLOAT) + emit_p[y].get(obs[t],MIN_FLOAT) ,y0) for y0 in prev_states])
+            (prob,state ) = max([(V[t-1][y0] + trans_p[y0].get(y,MIN_INF) + emit_p[y].get(obs[t],MIN_FLOAT) ,y0) for y0 in prev_states])
             V[t][y] =prob
             mem_path[t][y] = state