diff --git a/build.sh b/build.sh
index 1b088d623..add6305de 100755
--- a/build.sh
+++ b/build.sh
@@ -4,4 +4,5 @@ if [ ! -d ".build" ]; then
 fi
 . .build/bin/activate
 pip install -U -r requirements.txt
+pip install -U tox
 python -m tox $@