MVC File Upload - Access to the path is denied

An interesting problem occurred yesterday when I was trying to upload a file to the server. I was getting:

Access to the path '' is denied

The problem wasn't actually caused by the file upload but by my attempt to create a directory to put that file there. After reading a bunch of answers on StackOverflow, most of which related to IIS Pool modifications... Surely, I can't be that complicated. Well it wasn't:

Spot the difference?

This doesn't work:

This does work:

It seems HttpContext.Server.MapPath("~/Images/" + Object1.Object1Id); makes all the difference.