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

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