net.lisp: Improve error in `ipmask'.

diff --git a/net.lisp b/net.lisp
index 7cd7e6d..1e32684 100644 (file)
--- a/net.lisp
+++ b/net.lisp
@@ -244,7 +244,7 @@
     (let ((w (ipaddr-width addr)))
       (if (<= 0 mask w)
          (integer-netmask w mask)
-         (error "Mask out of range.")))))
+         (error "Prefix length out of range.")))))
 
 (export 'mask-ipaddr)
 (defun mask-ipaddr (addr mask)