diff --git a/src/bvh/v2/index.h b/src/bvh/v2/index.h
index beec4ae..7120031 100644
--- a/src/bvh/v2/index.h
+++ b/src/bvh/v2/index.h
@@ -36,6 +36,7 @@ struct Index {
     static constexpr size_t bits = Bits;
     static constexpr size_t prim_count_bits = PrimCountBits;
     static constexpr Type max_prim_count = make_bitmask<Type>(prim_count_bits);
+    static constexpr Type max_first_id   = make_bitmask<Type>(bits - prim_count_bits);
 
     static_assert(PrimCountBits < Bits);
 
@@ -52,29 +53,31 @@ struct Index {
     BVH_ALWAYS_INLINE bool is_leaf() const { return prim_count() != 0; }
     BVH_ALWAYS_INLINE bool is_inner() const { return !is_leaf(); }
 
-    BVH_ALWAYS_INLINE void set_first_id(Type first_id) {
+    BVH_ALWAYS_INLINE void set_first_id(size_t first_id) {
         *this = Index { first_id, prim_count() };
     }
 
-    BVH_ALWAYS_INLINE void set_prim_count(Type prim_count) {
+    BVH_ALWAYS_INLINE void set_prim_count(size_t prim_count) {
         *this = Index { first_id(), prim_count };
     }
 
-    static BVH_ALWAYS_INLINE Index make_leaf(Type first_prim, Type prim_count) {
+    static BVH_ALWAYS_INLINE Index make_leaf(size_t first_prim, size_t prim_count) {
         assert(prim_count != 0);
         return Index { first_prim, prim_count };
     }
 
-    static BVH_ALWAYS_INLINE Index make_inner(Type first_child) {
+    static BVH_ALWAYS_INLINE Index make_inner(size_t first_child) {
         return Index { first_child, 0 };
     }
 
 private:
-    explicit Index(Type first_id, Type prim_count)
-        : value((first_id << prim_count_bits) | (prim_count & max_prim_count))
+    explicit Index(size_t first_id, size_t prim_count)
+        : Index(
+            (static_cast<Type>(first_id) << prim_count_bits) |
+            (static_cast<Type>(prim_count) & max_prim_count))
     {
-        assert(first_id <= make_bitmask<Type>(Bits - PrimCountBits));
-        assert(prim_count <= max_prim_count);
+        assert(first_id   <= static_cast<size_t>(max_first_id));
+        assert(prim_count <= static_cast<size_t>(max_prim_count));
     }
 };