Next & Previous Post Thumbnail

I am building a custom theme and i’m using the post thumbnail function to show next and previous posts in my sidebar.

Here is the code i’m using

[Code moderated as per the Forum Rules. Please use the pastebin]

I’m having a issue when I get the the forth post. It seems to just show an empty image which I dont want.

Any ideas on how to fix it?

Thanks

– – – – – – –

http://itdrewitself.com/illustration/landfill-editions/