diff --git a/tests/test_harness.c b/tests/test_harness.c
index ad55f9d..7910bfb 100644
--- a/tests/test_harness.c
+++ b/tests/test_harness.c
@@ -46,6 +46,9 @@ struct image_data_header
   /* The width and height.  */
   unsigned short width, height;
 
+  /* Padding.  */
+  unsigned short pad1;
+
   /* The stride.  */
   unsigned int stride;
 };