diff --git a/usr.sbin/bsdinstall/scripts/script b/usr.sbin/bsdinstall/scripts/script
--- a/usr.sbin/bsdinstall/scripts/script
+++ b/usr.sbin/bsdinstall/scripts/script
@@ -98,6 +98,7 @@
     "$SCRIPT" $TMPDIR/bsdinstall-installscript-setup \
     >$TMPDIR/bsdinstall-installscript-preamble
 
+: ${BSDINSTALL_LOG="${debugFile#+}"}
 . $TMPDIR/bsdinstall-installscript-preamble
 export BSDINSTALL_DISTDIR