handle #f when printing mailbox object

@@ -58,6 +58,13 @@ mu_scm_mailbox_print (SCM mailbox_smob, SCM port, scm_print_state * pstate)
 
   scm_puts ("#<mailbox ", port);
 
+  if (mailbox_smob == SCM_BOOL_F)
+    {
+      /* mu_mailbox.* functions may return #f */
+      scm_puts ("#f", port);
+    }
+  else
+    {
       p = url_to_string (url);
       if (p)
 	{
@@ -70,6 +77,7 @@ mu_scm_mailbox_print (SCM mailbox_smob, SCM port, scm_print_state * pstate)
 	}
       else
 	scm_puts ("uninitialized", port);
+    }
   scm_puts (">", port);
 
   return 1;