diff --git a/src/calibre.hpp b/src/calibre.hpp
index 83d20bf..5bc1351 100644
--- a/src/calibre.hpp
+++ b/src/calibre.hpp
@@ -118,11 +118,25 @@ namespace calibre {
                bool StartObject() {
                        // We only want the book object @ depth 0 (pre-increment), not any of the nested ones.
                        if (depth++ > 0) {
+                               // NOTE: If we encounter an object as the value of a required_field, the input file is malformed,
+                               //       as none of our required_fields should ever be objects ;).
+                               //       We've seen examples where a required_field that should have been an array is instead described as an empty *object*,
+                               //       c.f., https://github.com/koreader/koreader/pull/11922#issuecomment-2152799500
+                               //       As we've currently got a key at the top of the Lua stack, one that we'll potentially never submit because of the heuristics,
+                               //       pop it *right now* to avoid leaving it dangling and unbalancing the Lua stack.
+                               if (required_field) {
+                                       lua_pop(L, 1);
+                                       // Also, unflag the field as required so as not to submit *anything* from that object, in case it isn't actually empty.
+                                       required_field = false;
+                               }
                                return true;
                        }
 
                        // We know exactly how many key-value pairs we'll need
                        lua_createtable(L, 0, required_fields.size());
+                       // Mark table as a JSON object (used when dumping back to JSON)
+                       luaL_getmetatable(L, "json.object");
+                       lua_setmetatable(L, -2);
 
                        // Switch to the object (i.e., hash) context
                        stack_.push_back(context_);
@@ -160,6 +174,9 @@ namespace calibre {
                        }
 
                        lua_newtable(L);
+                       // Mark table as a JSON array
+                       luaL_getmetatable(L, "json.array");
+                       lua_setmetatable(L, -2);
 
                        // Switch to the array context
                        stack_.push_back(context_);
@@ -205,7 +222,7 @@ namespace calibre {
                                fn_(L, this);
                        }
 
-                       int index_;
+                       rapidjson::SizeType index_;
                        void(*fn_)(lua_State* L, Ctx* ctx);
 
                private:
@@ -229,7 +246,7 @@ namespace calibre {
                };
 
                mutable bool required_field;
-               size_t depth;
+               rapidjson::SizeType depth;
                const std::unordered_set<std::string> required_fields {
                        "authors",
                        "last_modified",
diff --git a/src/values.hpp b/src/values.hpp
index c1e4b72..9afe8bb 100644
--- a/src/values.hpp
+++ b/src/values.hpp
@@ -198,7 +198,7 @@ namespace values {
                                fn_(L, this);
                        }
 
-                       int index_;
+                       rapidjson::SizeType index_;
                        void(*fn_)(lua_State* L, Ctx* ctx);
                private:
                        explicit Ctx(void(*f)(lua_State* L, Ctx* ctx)) : index_(0), fn_(f) {}