diff --git a/src/lune/scheduler/state.rs b/src/lune/scheduler/state.rs
index 6050810..f2df7ee 100644
--- a/src/lune/scheduler/state.rs
+++ b/src/lune/scheduler/state.rs
@@ -1,30 +1,34 @@
 use std::{
-    cell::RefCell,
     collections::HashMap,
-    sync::atomic::{AtomicBool, AtomicU8, AtomicUsize, Ordering},
+    sync::{
+        atomic::{AtomicBool, AtomicU8, AtomicUsize, Ordering},
+        Arc, Mutex,
+    },
 };
 
 use mlua::Error as LuaError;
 
 use super::SchedulerThreadId;
 
+/**
+    Internal state for a [`Scheduler`].
+
+    This scheduler state uses atomic operations for everything
+    except lua error storage, and is completely thread safe.
+*/
 #[derive(Debug, Default)]
 pub struct SchedulerState {
     exit_state: AtomicBool,
     exit_code: AtomicU8,
     num_resumptions: AtomicUsize,
     num_errors: AtomicUsize,
-    // TODO: Use Arc<Mutex<T>> to make these thread and borrow safe
-    thread_id: RefCell<Option<SchedulerThreadId>>,
-    thread_errors: RefCell<HashMap<SchedulerThreadId, LuaError>>,
+    thread_id: Arc<Mutex<Option<SchedulerThreadId>>>,
+    thread_errors: Arc<Mutex<HashMap<SchedulerThreadId, LuaError>>>,
 }
 
 impl SchedulerState {
     /**
         Creates a new scheduler state.
-
-        This scheduler state uses atomic operations for everything
-        except lua resumption errors, and is completely thread safe.
     */
     pub fn new() -> Self {
         Self::default()
@@ -80,18 +84,33 @@ impl SchedulerState {
         Gets the currently running lua scheduler thread id, if any.
     */
     pub fn get_current_thread_id(&self) -> Option<SchedulerThreadId> {
-        *self.thread_id.borrow()
+        *self
+            .thread_id
+            .lock()
+            .expect("Failed to lock current thread id")
     }
 
     /**
         Sets the currently running lua scheduler thread id.
 
-        This should be set to `Some(id)` just before resuming a lua
-        thread, and `None` while no lua thread is being resumed.
+        This must be set to `Some(id)` just before resuming a lua thread,
+        and `None` while no lua thread is being resumed. If set to `Some`
+        while the current thread id is also `Some`, this will panic.
+
+        Must only be set once per thread id, although this
+        is not checked at runtime for performance reasons.
     */
     pub fn set_current_thread_id(&self, id: Option<SchedulerThreadId>) {
-        self.thread_id.replace(id);
         self.num_resumptions.fetch_add(1, Ordering::Relaxed);
+        let mut thread_id = self
+            .thread_id
+            .lock()
+            .expect("Failed to lock current thread id");
+        assert!(
+            id.is_none() || thread_id.is_none(),
+            "Current thread id can not be overwritten"
+        );
+        *thread_id = id;
     }
 
     /**
@@ -100,7 +119,11 @@ impl SchedulerState {
         Note that this removes the error from the scheduler state completely.
     */
     pub fn get_thread_error(&self, id: SchedulerThreadId) -> Option<LuaError> {
-        self.thread_errors.borrow_mut().remove(&id)
+        let mut thread_errors = self
+            .thread_errors
+            .lock()
+            .expect("Failed to lock thread errors");
+        thread_errors.remove(&id)
     }
 
     /**
@@ -109,6 +132,10 @@ impl SchedulerState {
         Note that this will replace any already existing [`LuaError`].
     */
     pub fn set_thread_error(&self, id: SchedulerThreadId, err: LuaError) {
-        self.thread_errors.borrow_mut().insert(id, err);
+        let mut thread_errors = self
+            .thread_errors
+            .lock()
+            .expect("Failed to lock thread errors");
+        thread_errors.insert(id, err);
     }
 }