It is because WP_Customize_Media_Control
requires an attachment ID so the default image must exist in the Media Library.
If you change the default value to an attachment ID it will work e.g:
$wp_customize->add_setting( 'my-custom-image', array(
'default' => '100',
'sanitize_callback' => 'absint',
) );
-
This reply was modified 7 years, 9 months ago by seothemes.