diff --git a/Makefile b/Makefile
index 55e8177..949a8f0 100644
--- a/Makefile
+++ b/Makefile
@@ -188,7 +188,8 @@ ${bld}/os.32k: example-os/os.asm ${bld}/check_makefile
 	nasm -w+macro-params -w+macro-selfref -w+orphan-labels -w+gnu-elf-extensions -o $@ $<
 
 ${bld}/os.iso: ${bld}/iso_files/os.zip ${bld}/iso_files/boot/iso_boot.sys ${bld}/check_makefile
-	cp -a -- ${bld}/iso_files ${bld}/iso_files.tmp
+	! test -d ${bld}/iso_files.tmp
+	cp -a -T -- ${bld}/iso_files ${bld}/iso_files.tmp
 	find ${bld}/iso_files.tmp -depth -exec touch -t ${commit_timestamp} '{}' ';'
 	faketime -f ${commit_faketime} mkisofs \
 	 --input-charset utf-8 \