diff --git a/test/rdp-access/rdp-access b/test/rdp-access/rdp-access
index dc0ad27..c2bb475 100755
--- a/test/rdp-access/rdp-access
+++ b/test/rdp-access/rdp-access
@@ -198,6 +198,9 @@ start_ipv6_forward() {
     if ipv6_listener_active; then
         # Do not kill an unrelated listener.  The operator can inspect it via
         # status; down only stops a PID that this helper started and verified.
+        if [ "$RDP_ACCESS_IPV6_FORWARD" = always ]; then
+            fail "IPv6 port $RDP_ACCESS_HTTP_PORT is already occupied by an unowned listener"
+        fi
         printf 'rdp-access: IPv6 port %s is already occupied; retaining the existing listener\n' \
             "$RDP_ACCESS_HTTP_PORT" >&2
         return 0