diff --git a/diffcore-order.c b/diffcore-order.c
index 23e93852d8..a63f332d2e 100644
--- a/diffcore-order.c
+++ b/diffcore-order.c
@@ -10,28 +10,21 @@ static int order_cnt;
 
 static void prepare_order(const char *orderfile)
 {
-	int fd, cnt, pass;
+	int cnt, pass;
+	struct strbuf sb = STRBUF_INIT;
 	void *map;
 	char *cp, *endp;
-	struct stat st;
-	size_t sz;
+	ssize_t sz;
 
 	if (order)
 		return;
 
-	fd = open(orderfile, O_RDONLY);
-	if (fd < 0)
-		return;
-	if (fstat(fd, &st)) {
-		close(fd);
-		return;
-	}
-	sz = xsize_t(st.st_size);
-	map = mmap(NULL, sz, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
-	close(fd);
-	if (map == MAP_FAILED)
-		return;
+	sz = strbuf_read_file(&sb, orderfile, 0);
+	if (sz < 0)
+		die_errno(_("failed to read orderfile '%s'"), orderfile);
+	map = strbuf_detach(&sb, NULL);
 	endp = (char *) map + sz;
+
 	for (pass = 0; pass < 2; pass++) {
 		cnt = 0;
 		cp = map;
diff --git a/t/t4056-diff-order.sh b/t/t4056-diff-order.sh
index c39ec4121a..ae8036b73a 100755
--- a/t/t4056-diff-order.sh
+++ b/t/t4056-diff-order.sh
@@ -57,12 +57,38 @@ test_expect_success "no order (=tree object order)" '
 	test_cmp expect_none actual
 '
 
+test_expect_success 'missing orderfile' '
+	rm -f bogus_file &&
+	test_must_fail git diff -Obogus_file --name-only HEAD^..HEAD
+'
+
+test_expect_success POSIXPERM,SANITY 'unreadable orderfile' '
+	>unreadable_file &&
+	chmod -r unreadable_file &&
+	test_must_fail git diff -Ounreadable_file --name-only HEAD^..HEAD
+'
+
+test_expect_success 'orderfile is a directory' '
+	test_must_fail git diff -O/ --name-only HEAD^..HEAD
+'
+
 for i in 1 2
 do
 	test_expect_success "orderfile using option ($i)" '
 		git diff -Oorder_file_$i --name-only HEAD^..HEAD >actual &&
 		test_cmp expect_$i actual
 	'
+
+	test_expect_success PIPE "orderfile is fifo ($i)" '
+		rm -f order_fifo &&
+		mkfifo order_fifo &&
+		{
+			cat order_file_$i >order_fifo &
+		} &&
+		git diff -O order_fifo --name-only HEAD^..HEAD >actual &&
+		wait &&
+		test_cmp expect_$i actual
+	'
 done
 
 test_done