diff --git a/requirements.txt b/requirements.txt
index 5dca80b..5f7ce1c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -118,7 +118,7 @@ nb-conda-kernels==2.2.2
 nbconvert==5.4.1
 nbformat==4.4.0
 networkx==2.1
-nltk==3.3
+nltk==3.6.3
 nose==1.3.7
 notebook==5.5.0
 numba==0.38.0