diff --git a/BookhoardAPI.lua b/BookhoardAPI.lua
index 8fc05d7..f818acf 100644
--- a/BookhoardAPI.lua
+++ b/BookhoardAPI.lua
@@ -1,5 +1,18 @@
 local json = require("json")
 local json_util = require("json.util")
+
+-- Recursively replace LuaJSON's truthy null-sentinel functions with nil
+-- (JSON null), so server payloads behave like normal Lua tables.
+local function strip_json_nulls(t)
+    for k, v in pairs(t) do
+        if type(v) == "function" then
+            t[k] = nil
+        elseif type(v) == "table" then
+            strip_json_nulls(v)
+        end
+    end
+    return t
+end
 local logger = require("logger")
 local ltn12 = require("ltn12")
 local socketutil = require("socketutil")
@@ -86,6 +99,11 @@ function BookhoardAPI:_request(method, path, body, timeouts)
         if response_body and #response_body > 0 then
             local decode_ok, data = pcall(json.decode, response_body)
             if decode_ok and type(data) == "table" then
+                -- LuaJSON decodes JSON null into a truthy sentinel FUNCTION,
+                -- which defeats every `if not x.field` nil-check downstream
+                -- (crash: "attempt to index local 'progress' (a function
+                -- value)"). Normalize sentinels to nil at the boundary.
+                strip_json_nulls(data)
                 return true, data
             end
             return true, response_body