diff --git a/src/serpent.lua b/src/serpent.lua
index 94284ed..fa570f0 100644
--- a/src/serpent.lua
+++ b/src/serpent.lua
@@ -1,4 +1,4 @@
-local n, v = "serpent", 0.279 -- (C) 2012-15 Paul Kulchenko; MIT License
+local n, v = "serpent", 0.28 -- (C) 2012-15 Paul Kulchenko; MIT License
 local c, d = "Paul Kulchenko", "Lua serializer and pretty printer"
 local snum = {[tostring(1/0)]='1/0 --[[math.huge]]',[tostring(-1/0)]='-1/0 --[[-math.huge]]',[tostring(0/0)]='0/0'}
 local badtype = {thread = true, userdata = true, cdata = true}
@@ -18,8 +18,8 @@ local function s(t, opts)
   local seen, sref, syms, symn = {}, {'local '..iname..'={}'}, {}, 0
   local function gensym(val) return '_'..(tostring(tostring(val)):gsub("[^%w]",""):gsub("(%d%w+)",
     -- tostring(val) is needed because __tostring may return a non-string value
-    function(s) if not syms[s] then symn = symn+1; syms[s] = symn end return syms[s] end)) end
-  local function safestr(s) return type(s) == "number" and (huge and snum[tostring(s)] or s)
+    function(s) if not syms[s] then symn = symn+1; syms[s] = symn end return tostring(syms[s]) end)) end
+  local function safestr(s) return type(s) == "number" and tostring(huge and snum[tostring(s)] or s)
     or type(s) ~= "string" and tostring(s) -- escape NEWLINE/010 and EOF/026
     or ("%q"):format(s):gsub("\010","n"):gsub("\026","\\026") end
   local function comment(s,l) return comm and (l or 0) < comm and ' --[['..tostring(s)..']]' or '' end
@@ -32,7 +32,7 @@ local function s(t, opts)
     return (path or '')..(plain and path and '.' or '')..safe, safe end
   local alphanumsort = type(opts.sortkeys) == 'function' and opts.sortkeys or function(k, o, n) -- k=keys, o=originaltable, n=padding
     local maxn, to = tonumber(n) or 12, {number = 'a', string = 'b'}
-    local function padnum(d) return ("%0"..maxn.."d"):format(d) end
+    local function padnum(d) return ("%0"..tostring(maxn).."d"):format(tonumber(d)) end
     table.sort(k, function(a,b)
       -- sort numeric keys first: k[key] is not nil for numerical keys
       return (k[a] ~= nil and 0 or to[type(a)] or 'z')..(tostring(a):gsub("%d+",padnum))