diff --git a/script/run-in-env.sh b/script/run-in-env.sh
index c71738a017b5163ad71ceb778de22d90b1b500b7..1c7f76ccc1f1dfcb0c3590c72bc21dfdba30225d 100755
--- a/script/run-in-env.sh
+++ b/script/run-in-env.sh
@@ -13,7 +13,7 @@ if [ -s .python-version ]; then
     export PYENV_VERSION
 fi
 
-if [ -n "${VIRTUAL_ENV}" ] && [ -f "${VIRTUAL_ENV}/bin/activate" ]; then
+if [ -n "${VIRTUAL_ENV-}" ] && [ -f "${VIRTUAL_ENV}/bin/activate" ]; then
   . "${VIRTUAL_ENV}/bin/activate"
 else
   # other common virtualenvs