diff --git a/requirements.txt b/requirements.txt
index c52024f7..3ad9228d 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -46,7 +46,7 @@ mock==2.0.0
 nbconvert==5.4.0
 nbformat==4.4.0
 networkx==1.11
-nltk==3.1
+nltk==3.6.3
 normality==0.2.4
 notebook==5.7.0
 numpy==1.13.3