class-frontend.php on line 406

This worked for me

Line 406:
$post->post_date = sprintf( "%04d-%02d-%02d 00:00:00", get_query_var( 'year' ), get_query_var( 'monthnum' ), get_query_var( 'day' ) );
Change to
if (is_object( $post)){{$post->post_date = sprintf( "%04d-%02d-%02d 00:00:00", get_query_var( 'year' ), get_query_var( 'monthnum' ), get_query_var( 'day' ) ); }

So it is now 5 months later, just wanted to add that this bug is still filling up error_log at my site:

[02-Mar-2014 05:12:02 UTC] PHP Warning:  Creating default object from empty value in /.../wp-content/plugins/wordpress-seo/frontend/class-frontend.php on line 411

Hoping for an eventual fix…

This has been fixed and the fix will be included in the next release (v1.5.3) ??

By the way, do realize that that bit of code was only ran when on a 404-page, so if your log was filling up, that must mean you were getting a lot of 404’s, which may be something you’d want to take a look at.