diff --git a/requirements.txt b/requirements.txt
index 802f8d4..d94295e 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -109,7 +109,7 @@ networkx==2.1
 neurdflib==5.0.0.post1
 nibabel==2.4.1
 nipype==1.2.0
-nltk==3.3
+nltk==3.6.3
 nose==1.3.7
 notebook==5.5.0
 numba==0.38.0