diff --git a/ch06/requirements.txt b/ch06/requirements.txt
index bbb3403..acdb642 100644
--- a/ch06/requirements.txt
+++ b/ch06/requirements.txt
@@ -1,6 +1,6 @@
 keras==2.1.6
 tensorflow==1.8.0
 h5py==2.7.1
-nltk==3.2.5
+nltk==3.6.3
 sklearn==0.0
 matplotlib==2.1.1
diff --git a/ch06/requirements_gpu.txt b/ch06/requirements_gpu.txt
index ae4b1a0..d5a2e37 100644
--- a/ch06/requirements_gpu.txt
+++ b/ch06/requirements_gpu.txt
@@ -1,6 +1,6 @@
 keras==2.1.6
 tensorflow-gpu==1.8.0
 h5py==2.7.1
-nltk==3.2.5
+nltk==3.6.3
 sklearn==0.0
 matplotlib==2.1.1