table index is nil about nginx-lua-prometheus HOT 5 CLOSED

/ # luarocks list

Rocks installed for Lua 5.1
---------------------------

lua-cjson
   2.1.0.10-1 (installed) - /usr/local/lib/luarocks/rocks-5.1

nginx-lua-prometheus
   0.20221218-1 (installed) - /usr/local/lib/luarocks/rocks-5.1

/ # cat -n /usr/local/share/lua/5.1/prometheus.lua | sed -n '370,390p'
   370
   371	  if self.lookup_size >= self.lookup_max_size then
   372	    self.lookup_size = 0
   373	    self.lookup = {}
   374	  end
   375
   376	  local t = self.lookup
   377	  if label_values then
   378	    -- Don't use ipairs here to avoid inner loop generates trace first
   379	    -- Otherwise the inner for loop below is likely to get JIT compiled before
   380	    -- the outer loop which include `lookup_or_create`, in this case the trace
   381	    -- for outer loop will be aborted. By not using ipairs, we will be able to
   382	    -- compile longer traces as possible.
   383	    local label
   384	    for i=1, self.label_count do
   385	      label = label_values[i]
   386	      if not t[label] then
   387	        t[label] = {}
   388	      end
   389	      t = t[label]
   390	    end