diff --git a/cache.lisp b/cache.lisp
index bb425e279df7c3f88220a8d1946f46704faf8bea..eb5754342dca7ee23bf5cfb4b47978ff9e7cc662 100644
--- a/cache.lisp
+++ b/cache.lisp
@@ -59,6 +59,7 @@
       (set-asdf-cache-entry `(get-file-stamp ,namestring) (list stamp))))
 
   (defun get-file-stamp (file)
-    (let ((namestring (normalize-namestring file)))
-      (do-asdf-cache `(get-file-stamp ,namestring) (compute-file-stamp namestring)))))
+    (when file
+      (let ((namestring (normalize-namestring file)))
+        (do-asdf-cache `(get-file-stamp ,namestring) (compute-file-stamp namestring))))))