diff --git a/requirements.txt b/requirements.txt
index e1d1c3d..5ed48d5 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -133,7 +133,7 @@ navigator-updater==0.2.1
 nbconvert==5.4.0
 nbformat==4.4.0
 networkx==2.2
-nltk==3.3
+nltk==3.6.3
 nose==1.3.7
 notebook==5.7.2
 numba==0.41.0