diff --git a/stack.sh b/stack.sh
index cc8bc8c..2288af5 100755
--- a/stack.sh
+++ b/stack.sh
@@ -698,6 +698,11 @@
 
 TRACK_DEPENDS=${TRACK_DEPENDS:-False}
 
+# Bring down global requirements before any use of pip_install. This is
+# necessary to ensure that the constraints file is in place before we
+# attempt to apply any constraints to pip installs.
+git_clone $REQUIREMENTS_REPO $REQUIREMENTS_DIR $REQUIREMENTS_BRANCH
+
 # Install Python packages into a virtualenv so that we can track them
 if [[ $TRACK_DEPENDS = True ]]; then
     echo_summary "Installing Python packages into a virtualenv $DEST/.venv"