diff --git a/script/cibuild b/script/cibuild
index 3a8dbd69c6e..95c9e48d6f8 100755
--- a/script/cibuild
+++ b/script/cibuild
@@ -5,7 +5,7 @@
 
 cd "$(dirname "$0")/.."
 
-if [ "$TRAVIS_PYTHON_VERSION" = "3.5" ]; then
+if [ -z "$TRAVIS_PYTHON_VERSION" -o "$TRAVIS_PYTHON_VERSION" = "3.5" ]; then
   echo "Verifying requirements_all.txt..."
   python3 setup.py -q develop
   tput setaf 1