diff --git a/nbsphinx.py b/nbsphinx.py
index e41d3de7..ae5cafce 100644
--- a/nbsphinx.py
+++ b/nbsphinx.py
@@ -41,6 +41,7 @@
 import nbconvert
 import nbformat
 import sphinx
+import sphinx.errors
 import traitlets
 
 _ipynbversion = 4