diff --git a/requirements.txt b/requirements.txt
index 17a2a13..5ecb458 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -96,7 +96,7 @@ joblib==0.13.2
 numexpr==2.6.9
 
 # Optional: these libraries can be useful in chapter 3, exercise 4.
-nltk==3.4
+nltk==3.4.5
 urlextract==0.9
 
 # Needed in chapter 19.