Comment by _ph_

The error was just bad, it could have been more explicit, and by now SBCL even issues a compile-time error for this problem. But you have a very valid point: the error was shown on the first testing of the function. And it is a very underappreciated feature of Common Lisp, that you can test almost every function by itself, you don't have to build and run an executable, you can, as the blog author did, test the single function and very quickly get to the problem.