[Minor] Fix Lua 5.1 compatibility

Issue: #2713

diff --git a/lualib/lua_util.lua b/lualib/lua_util.lua
index b12a258f5be8a7fb9d6123b095e19e4e686fcdb7..34bafcc4ba323bb91e4ba4de85b5b7dcf81394da 100644 (file)
--- a/lualib/lua_util.lua
+++ b/lualib/lua_util.lua
@@ -593,7 +593,8 @@ exports.extract_specific_urls = function(params_or_task, lim, need_emails, filte
   if params.prefix then
     cache_key = params.prefix
   else
-    cache_key = string.format('sp_urls_%d%s', params.limit, params.need_emails)
+    cache_key = string.format('sp_urls_%d%s', params.limit,
+        tostring(params.need_emails))
   end