diff --git a/stack.sh b/stack.sh
index 591c0dc..4069509 100755
--- a/stack.sh
+++ b/stack.sh
@@ -683,6 +683,9 @@
 echo_summary "Installing package prerequisites"
 source $TOP_DIR/tools/install_prereqs.sh
 
+# Normalise USE_CONSTRAINTS
+USE_CONSTRAINTS=$(trueorfalse False USE_CONSTRAINTS)
+
 # Configure an appropriate Python environment
 if [[ "$OFFLINE" != "True" ]]; then
     PYPI_ALTERNATIVE_URL=${PYPI_ALTERNATIVE_URL:-""} $TOP_DIR/tools/install_pip.sh