diff --git a/requirements_docs.txt b/requirements_docs.txt
index f3dc318f3f..99c550a018 100644
--- a/requirements_docs.txt
+++ b/requirements_docs.txt
@@ -7,7 +7,7 @@ Pyro4==4.77
 annoy==1.16.2
 scikit-learn==0.21.3
 plotly==4.2.1
-nltk==3.4.5
+nltk==3.9
 testfixtures==6.10.0
 statsmodels==0.10.1
 pyemd==0.5.1