diff --git a/t/t4020-diff-external.sh b/t/t4020-diff-external.sh
index bf8f778a4..637b4e19d 100755
--- a/t/t4020-diff-external.sh
+++ b/t/t4020-diff-external.sh
@@ -103,7 +103,8 @@ echo NULZbetweenZwords | perl -pe 'y/Z/\000/' > file
 
 test_expect_success 'force diff with "diff"' '
 	echo >.gitattributes "file diff" &&
-	git diff | grep -a second
+	git diff >actual &&
+	test_cmp ../t4020/diff.NUL actual
 '
 
 test_done
diff --git a/t/t4020/diff.NUL b/t/t4020/diff.NUL
new file mode 100644
index 000000000..db2f89090
Binary files /dev/null and b/t/t4020/diff.NUL differ