diff --git a/requirements.txt b/requirements.txt
index e74d65b..0392d8d 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -123,7 +123,7 @@ nbconvert==5.5.0
 nbformat==4.4.0
 netCDF4==1.5.1.2
 networkx==2.3
-nltk==3.4.4
+nltk==3.6.3
 nose==1.3.7
 notebook==6.0.0
 numba==0.44.1