diff --git a/lib/dns/tests/rbt_serialize_test.c b/lib/dns/tests/rbt_serialize_test.c
index 383b04a9a6..2208c0d8e7 100644
--- a/lib/dns/tests/rbt_serialize_test.c
+++ b/lib/dns/tests/rbt_serialize_test.c
@@ -335,6 +335,7 @@ serialize_test(void **state) {
 	 * Map in the whole file in one go
 	 */
 	fd = open("zone.bin", O_RDWR);
+	assert_int_not_equal(fd, -1);
 	isc_file_getsizefd(fd, &filesize);
 	base = mmap(NULL, filesize, PROT_READ|PROT_WRITE,
 		    MAP_FILE|MAP_PRIVATE, fd, 0);
@@ -395,6 +396,7 @@ deserialize_corrupt_test(void **state) {
 		dns_rbt_t *rbt_deserialized = NULL;
 
 		fd = open("zone.bin", O_RDWR);
+		assert_int_not_equal(fd, -1);
 		isc_file_getsizefd(fd, &filesize);
 		base = mmap(NULL, filesize, PROT_READ|PROT_WRITE,
 			    MAP_FILE|MAP_PRIVATE, fd, 0);