Could this be the BUILT-IN-SYSTEM-P check? Faré recently added this block to the code for wrangling system versions: (let ((sysfile (system-source-file (component-system component)))) (when (and (typep component 'system) (not bspp)) (setf (builtin-system-p component) (lisp-implementation-pathname-p sysfile))) (setf version (normalize-version version :component name :parent parent :pathname sysfile))) Question: what version of ASDF are you testing on? The version of the code above dates from Feb 7. Any chance you are testing an earlier version? Faré: it looks like BSPP here is built-in-system-p-supplied-p, yes? If so, it looks like this block is trying to guess whether a system is built in. If this is the source of the error, though, I don't see how (TYPEP NIL 'SYSTEM) could be passing. Thanks, r