diff --git a/pip-sample/requirements.txt b/pip-sample/requirements.txt
index 74d37475..5d5371c5 100644
--- a/pip-sample/requirements.txt
+++ b/pip-sample/requirements.txt
@@ -30,7 +30,7 @@ matplotlib==2.2.2
 mistune==0.8.3
 nbconvert==5.4.0
 nbformat==4.4.0
-nltk==3.3
+nltk==3.6.6
 notebook==5.7.0
 numpy==1.15.3
 oauthlib==2.1.0