diff --git a/all-your-base/all-your-base.fasl b/all-your-base/all-your-base.fasl
index f474b64..1f9dc5d 100644
Binary files a/all-your-base/all-your-base.fasl and b/all-your-base/all-your-base.fasl differ
diff --git a/all-your-base/all-your-base.lisp b/all-your-base/all-your-base.lisp
index 3a6b85d..bcf3d60 100644
--- a/all-your-base/all-your-base.lisp
+++ b/all-your-base/all-your-base.lisp
@@ -4,13 +4,10 @@
 
 (in-package :all-your-base)
 
-; `rebase` needs to return NIL sometimes. For the cases where one bad list
-; item yields NIL, we need `conv` to return multiple values: (ok, val).
-; Then we can check the ok before proceeding to `unconv`.`
 (defun conv (l base)
-    (if (null l)
-        0
-        (+ (* (first l) (expt base (- (length l) 1))) (conv (rest l) base))))
+  (if (null l) 
+      0
+      (+ (* (first l) (expt base (- (length l) 1))) (conv (rest l) base))))
 
 (defun calc-start-exponent (num base x)
     (if (< num (expt base (1+ x)))
@@ -29,4 +26,8 @@
               (append (list q) (unconv r base (- x 1)))))))
 
 (defun rebase (l source destination)
-    (unconv (conv l source) destination -1))
+    (cond ((< source 2) nil)
+          ((< destination 2) nil)
+          ((some #'(lambda (x) (< x 0)) l) nil)
+          ((some #'(lambda (x) (>= x source)) l) nil)
+          (t (unconv (conv l source) destination -1))))