diff --git a/tests/capture.py b/tests/capture.py
index 2ad9385..a7b7583 100755
--- a/tests/capture.py
+++ b/tests/capture.py
@@ -36,10 +36,12 @@ c_buf = c_img.get_data()
 
 for x in range(width):
     for y in range(height):
+        # The upper byte is don't care, but the location depends on endianness,
+        # so just set all of them.
         c_buf[y * c_rowstride + x * 4 + 0] = buf[y * width + x]
         c_buf[y * c_rowstride + x * 4 + 1] = buf[y * width + x]
         c_buf[y * c_rowstride + x * 4 + 2] = buf[y * width + x]
-        # Byte 4 is don't care
+        c_buf[y * c_rowstride + x * 4 + 3] = buf[y * width + x]
 
 c_img.mark_dirty()
 c_img.write_to_png(sys.argv[1])