diff --git a/gitlab/build.sh b/gitlab/build.sh
index 10cfb63a8044ef0c9c80019a0ec77270186e8a8e..3af02af73d064d7edaae42422ad6374f3c2d2bee 100755
--- a/gitlab/build.sh
+++ b/gitlab/build.sh
@@ -20,7 +20,7 @@ if [ -x ./bin/bob_dbmanage.py ]; then
 fi
 
 if [ -d ./doc ]; then
-  run_cmd ./bin/sphinx-build doc sphinx
+  run_cmd ./bin/sphinx-build -W doc sphinx
 fi
 
 if [ -z "${WHEEL_TAG}" ]; then