diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 01fa053f..842c1ceb 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -381,7 +381,9 @@ jobs:
           overwrite: true
       - name: Build
         if: ${{ !matrix.no-build }}
-        run: cargo build ${{ matrix.optimization && '--release' || '' }} --target ${{ matrix.target }} --no-default-features --features ${{ matrix.features }}
+        run: |
+          cargo clean
+          cargo build ${{ matrix.optimization && '--release' || '' }} --target ${{ matrix.target }} --no-default-features --features ${{ matrix.features }}
       - name: Test
         if: ${{ !matrix.no-build && !matrix.no-test }}
         timeout-minutes: 12
diff --git a/core/src/class/id.rs b/core/src/class/id.rs
index 52013919..e0837481 100644
--- a/core/src/class/id.rs
+++ b/core/src/class/id.rs
@@ -1,13 +1,10 @@
 use rquickjs_sys::JSRuntime;
 
-use crate::qjs;
-use std::{
-    collections::HashMap,
-    sync::{
-        atomic::{AtomicUsize, Ordering},
-        OnceLock, RwLock,
-    },
+use crate::{
+    qjs,
+    runtime::opaque::{ClassIdKey, Opaque},
 };
+use std::sync::atomic::{AtomicUsize, Ordering};
 
 /// The type of identifier of class
 #[cfg_attr(feature = "doc-cfg", doc(cfg(feature = "classes")))]
@@ -15,14 +12,7 @@ pub struct ClassId {
     type_id: AtomicUsize,
 }
 
-#[derive(Eq, Hash, PartialEq)]
-struct ClassIdKey(*mut JSRuntime, usize);
-
-unsafe impl Sync for ClassIdKey {}
-unsafe impl Send for ClassIdKey {}
-
 static CLASS_ID_COUNTER: AtomicUsize = AtomicUsize::new(1);
-static CLASS_ID_MAP: OnceLock<RwLock<HashMap<ClassIdKey, qjs::JSClassID>>> = OnceLock::new();
 
 impl ClassId {
     /// Create a new class id.
@@ -39,18 +29,15 @@ impl ClassId {
     pub fn get(&self, rt: *mut JSRuntime) -> qjs::JSClassID {
         let type_id = self.init_type_id();
         let key = ClassIdKey(rt, type_id);
-        let class_id_lock = CLASS_ID_MAP.get_or_init(|| RwLock::new(HashMap::new()));
-        if let Some(class_id) = class_id_lock.read().unwrap().get(&key) {
-            return *class_id;
-        }
 
-        let mut read_lock = class_id_lock.write().unwrap();
-        let mut id = 0;
-        unsafe { qjs::JS_NewClassID(rt, &mut id) };
+        let opaque = unsafe { &(*qjs::JS_GetRuntimeOpaque(rt).cast::<Opaque>()) };
 
-        read_lock.insert(key, id);
-
-        id
+        let id = opaque.get_class_id_map().entry(key).or_insert_with(|| {
+            let mut id = 0;
+            unsafe { qjs::JS_NewClassID(rt, &mut id) };
+            id
+        });
+        *id
     }
 
     /// Initialize the class ID.
diff --git a/core/src/runtime/opaque.rs b/core/src/runtime/opaque.rs
index 00c0f982..1fa4d9c5 100644
--- a/core/src/runtime/opaque.rs
+++ b/core/src/runtime/opaque.rs
@@ -1,3 +1,7 @@
+use rquickjs_sys::JSRuntime;
+
+use crate::qjs;
+
 use super::{
     userdata::{UserDataGuard, UserDataMap},
     InterruptHandler, UserData, UserDataError,
@@ -5,6 +9,7 @@ use super::{
 use std::{
     any::Any,
     cell::{Cell, UnsafeCell},
+    collections::HashMap,
     marker::PhantomData,
 };
 
@@ -17,6 +22,9 @@ use std::{
     task::{Context, Waker},
 };
 
+#[derive(Eq, Hash, PartialEq)]
+pub struct ClassIdKey(pub *mut JSRuntime, pub usize);
+
 /// Opaque book keeping data for Rust.
 pub(crate) struct Opaque<'js> {
     /// Used to carry a panic if a callback triggered one.
@@ -25,6 +33,8 @@ pub(crate) struct Opaque<'js> {
     /// The user provided interrupt handler, if any.
     interrupt_handler: UnsafeCell<Option<InterruptHandler>>,
 
+    class_id_map: UnsafeCell<HashMap<ClassIdKey, qjs::JSClassID>>,
+
     userdata: UserDataMap,
 
     #[cfg(feature = "futures")]
@@ -38,6 +48,7 @@ impl<'js> Opaque<'js> {
         Opaque {
             panic: Cell::new(None),
             interrupt_handler: UnsafeCell::new(None),
+            class_id_map: UnsafeCell::new(HashMap::new()),
             userdata: UserDataMap::default(),
             #[cfg(feature = "futures")]
             spawner: None,
@@ -50,6 +61,7 @@ impl<'js> Opaque<'js> {
         Opaque {
             panic: Cell::new(None),
             interrupt_handler: UnsafeCell::new(None),
+            class_id_map: UnsafeCell::new(HashMap::new()),
             userdata: UserDataMap::default(),
             #[cfg(feature = "futures")]
             spawner: Some(UnsafeCell::new(Spawner::new())),
@@ -120,4 +132,8 @@ impl<'js> Opaque<'js> {
     pub fn take_panic(&self) -> Option<Box<dyn Any + Send + 'static>> {
         self.panic.take()
     }
+
+    pub(crate) fn get_class_id_map(&self) -> &mut HashMap<ClassIdKey, u32> {
+        unsafe { &mut *self.class_id_map.get() }
+    }
 }
diff --git a/core/src/runtime/raw.rs b/core/src/runtime/raw.rs
index 20af36b2..5ddb1422 100644
--- a/core/src/runtime/raw.rs
+++ b/core/src/runtime/raw.rs
@@ -121,8 +121,8 @@ impl Drop for RawRuntime {
         unsafe {
             let ptr = qjs::JS_GetRuntimeOpaque(self.rt.as_ptr());
             let opaque: Box<Opaque> = Box::from_raw(ptr as *mut _);
+            qjs::JS_FreeRuntime(self.rt.as_ptr());
             mem::drop(opaque);
-            qjs::JS_FreeRuntime(self.rt.as_ptr())
         }
     }
 }