diff --git a/src/stim/diagram/gltf.cc b/src/stim/diagram/gltf.cc
index b78fdb64..0dc574ec 100644
--- a/src/stim/diagram/gltf.cc
+++ b/src/stim/diagram/gltf.cc
@@ -31,14 +31,14 @@ JsonObj GltfScene::_to_json_local() const {
 JsonObj GltfScene::to_json() {
     // Clear indices.
     visit([&](GltfId &item_id, const char *type, const std::function<JsonObj(void)> &to_json, uintptr_t abs_id) {
-        item_id.index = SIZE_MAX;
+        item_id.index = UINT64_MAX;
     });
 
     // Re-index.
     std::map<std::string, size_t> counts;
     visit([&](GltfId &item_id, const char *type, const std::function<JsonObj(void)> &to_json, uintptr_t abs_id) {
         auto &c = counts[type];
-        if (item_id.index == SIZE_MAX || item_id.index == c) {
+        if (item_id.index == UINT64_MAX || item_id.index == c) {
             item_id.index = c;
             c++;
         } else if (item_id.index > c) {
diff --git a/src/stim/diagram/gltf.h b/src/stim/diagram/gltf.h
index 1abfc97e..e6adb6d3 100644
--- a/src/stim/diagram/gltf.h
+++ b/src/stim/diagram/gltf.h
@@ -32,9 +32,9 @@ constexpr size_t GL_NEAREST = 9728;
 
 struct GltfId {
     std::string name;
-    size_t index;
+    uint64_t index;
 
-    GltfId(std::string name) : name(name), index(SIZE_MAX) {
+    GltfId(std::string name) : name(name), index(UINT64_MAX) {
     }
     GltfId() = delete;
 };
@@ -125,10 +125,10 @@ struct GltfBuffer {
 
 struct GltfSampler {
     GltfId id;
-    size_t magFilter;
-    size_t minFilter;
-    size_t wrapS;
-    size_t wrapT;
+    uint64_t magFilter;
+    uint64_t minFilter;
+    uint64_t wrapS;
+    uint64_t wrapT;
 
     void visit(const gltf_visit_callback &callback);
     JsonObj to_json() const;