Error of WDM (free) in W3C Validation Service

Hello!

I have a question about WordPress Download Manager (free version), please!
I use WDM in a custom post type with shortcode. Works nice, but when i try to validate my website with W3C , I recive an error.

This is the error:
“Bad value 2 for attribute rel on element a: Not an absolute IRI. The string 2 is not a registered keyword or absolute URL.”

And the HTML code looks like this:
Download6

And this is the code from my theme that display WDM shortcode:
ID, ‘download’, true) )
echo do_shortcode(get_post_meta($post->ID, ‘download’, $single = true));?>

Is there a way to fix this, please?

This is the HTML CODE.
http://s3.postimage.org/vbyeid6fn/html_code.png

I have attached another picture of W3C errors:

http://s12.postimage.org/nm7m27831/from_W3_C.png

I need to fix this, please, it’s important for me.