diff --git a/requirements/base.in b/requirements/base.in
index 99aae93..b37ea8a 100644
--- a/requirements/base.in
+++ b/requirements/base.in
@@ -3,3 +3,6 @@
 
 xblock[django]
 parsel
+# Dependency of parsel; see constraints.txt for more information about cssselect constraints.
+# Adding it here to ensure the constraint is honoured.
+cssselect