--- ugarit.scm
+++ ugarit.scm
@@ -38,13 +38,15 @@
     (sprintf "~AKiB" (inexact->exact (round (/ b 1024)))))
    (else
     (sprintf "~AB" b))))
 
 (define (percent a b)
-  (inexact->exact
-   (round
-    (* 100 (/ a b)))))
+  (if (zero? b)
+      100
+      (inexact->exact
+       (round
+        (* 100 (/ a b))))))
 
 (define progress-callback
   (let ((cpath #f)
         (csize #f)
         (bytes #f)