diff --git a/scripts/setup/machine-setup/lib/base.sh b/scripts/setup/machine-setup/lib/base.sh
index ee627b31..2549383c 100644
--- a/scripts/setup/machine-setup/lib/base.sh
+++ b/scripts/setup/machine-setup/lib/base.sh
@@ -121,12 +121,22 @@ load_answers() {
     [[ -n "$value" ]] || continue
     # The environment wins over what was saved.
     #
-    # Written as if/then rather than `[[ … ]] && assign`. That form returns
-    # non-zero when the test is false, and as the last statement in a loop body
-    # under `set -e` it takes the whole script down — which is exactly what
-    # happened the first time this ran with the variables already set in the
-    # environment: the file loaded fine, every value was already present, and the
-    # run died at "Step: unknown" before pre-flight.
+    # Written as if/then rather than `[[ … ]] && assign`.
+    #
+    # That form returns non-zero when the test is false. Harmless on its own —
+    # `set -e` exempts the left side of an && list — but here it is the last
+    # thing the case runs, the case is the last thing the loop body runs, and the
+    # loop is the last thing THE FUNCTION runs. So load_answers returned
+    # non-zero, and calling a function that returns non-zero is a plain command
+    # failure, which does end the script.
+    #
+    # It needed the answers file to exist AND the variables to be set already, so
+    # it only appeared when running with env overrides. The trap reported "Step:
+    # unknown" at a line inside this library, before pre-flight had run.
+    #
+    # The general rule this is an instance of: a function whose last statement
+    # can return non-zero fails when it is called, however innocuous the
+    # statement looks.
     case "$key" in
       MACHINE_ROLE) if [[ -z "${MACHINE_ROLE:-}" ]]; then MACHINE_ROLE="$value"; fi ;;
       SETUP_USERNAME) if [[ -z "${SETUP_USERNAME:-}" ]]; then SETUP_USERNAME="$value"; fi ;;