diff --git a/requirements.txt b/requirements.txt
index 2817d7f..dc1a50c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -50,7 +50,7 @@ moto==1.3.4
 msgpack==0.5.6
 msgpack-numpy==0.4.3.2
 murmurhash==1.0.1
-nltk==3.3
+nltk==3.6.6
 numpy==1.15.2
 numpydoc==0.8.0
 overrides==1.9