diff --git a/docs/environment.yml b/docs/environment.yml
index df62dff..b0c10b2 100644
--- a/docs/environment.yml
+++ b/docs/environment.yml
@@ -1,6 +1,7 @@
 name: ipython_docs
 dependencies:
 - python=3.6
+- prompt_toolkit>2
 - ipykernel
 - setuptools>=18.5
 - sphinx>=1.8