diff --git a/Archipelago.cpp b/Archipelago.cpp
index 47d4fc5..eed0f61 100644
--- a/Archipelago.cpp
+++ b/Archipelago.cpp
@@ -580,7 +580,11 @@ void AP_SendItems(AP_State* state, std::set<int64_t> const& locations) {
 
         Json::Value fake_msg;
         fake_msg[0]["cmd"] = "ReceivedItems";
-        fake_msg[0]["index"] = state->last_item_idx+1;
+        // size_t (unsigned long on LP64 macOS) is ambiguous across jsoncpp's
+        // UInt/Int64/UInt64 converting constructors because uint64_t is
+        // unsigned long long here (distinct from size_t). On Linux x64 uint64_t
+        // == unsigned long, so it resolved by exact match there. Cast explicitly.
+        fake_msg[0]["index"] = static_cast<Json::UInt64>(state->last_item_idx + 1);
         fake_msg[0]["items"] = Json::arrayValue;
         for (int64_t location_id : new_locations) {
             int64_t recv_item_id = state->location_to_item[location_id];
diff --git a/README.md b/README.md
index da7632c..969cf15 100644
--- a/README.md
+++ b/README.md
@@ -58,3 +58,8 @@ Clone the Repo recursively!
 - Enter the folder
 - `cmake .. -DWIN32=1` (If on MinGW, also add `-DMINGW=1`. If `zlib` is not installed add `-DUSE_ZLIB=OFF`)
 - `cmake --build .`
+## macOS
+- Create a folder `build`
+- `cd build`
+- `cmake ..` (builds for the host architecture; for a specific arch or a universal build add e.g. `-DCMAKE_OSX_ARCHITECTURES=arm64`, `x86_64`, or `"arm64;x86_64"`)
+- `cmake --build .`