diff --git a/requirements.txt b/requirements.txt
index a0e23ba..5ed90bd 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,7 +1,7 @@
 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT license.
 
-nltk==3.4
+nltk==3.6.6
 pattern
 numpy==1.14.0
 pytorch-pretrained-bert==0.5.1