diff --git a/mmtk/Cargo.toml b/mmtk/Cargo.toml
index 4329efd6..b610bb2f 100644
--- a/mmtk/Cargo.toml
+++ b/mmtk/Cargo.toml
@@ -20,7 +20,7 @@ lazy_static = "1.1"
 # - change branch
 # - change repo name
 # But other changes including adding/removing whitespaces in commented lines may break the CI.
-mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "4851e7ac7e8a7bd70ce65935c4ac1ff430cab6f1" }
+mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "4ebb6b68f54a55bf8794ea01db3f2d0f28d15f4d" }
 # Uncomment the following to build locally
 # mmtk = { path = "../repos/mmtk-core" }
 
diff --git a/mmtk/src/api.rs b/mmtk/src/api.rs
index e4d8fd64..5dd69ec6 100644
--- a/mmtk/src/api.rs
+++ b/mmtk/src/api.rs
@@ -120,7 +120,8 @@ pub extern "C" fn will_never_move(object: ObjectReference) -> bool {
 // We trust the worker pointer is valid.
 #[allow(clippy::not_unsafe_ptr_arg_deref)]
 pub extern "C" fn start_worker(tls: VMWorkerThread, worker: *mut GCWorker<OpenJDK>) {
-    memory_manager::start_worker::<OpenJDK>(tls, unsafe { worker.as_mut().unwrap() }, &SINGLETON)
+    let mut worker = unsafe { Box::from_raw(worker) };
+    memory_manager::start_worker::<OpenJDK>(tls, &mut worker, &SINGLETON)
 }
 
 #[no_mangle]
diff --git a/mmtk/src/collection.rs b/mmtk/src/collection.rs
index edaa92cb..59c8c978 100644
--- a/mmtk/src/collection.rs
+++ b/mmtk/src/collection.rs
@@ -49,9 +49,9 @@ impl Collection<OpenJDK> for VMCollection {
         }
     }
 
-    fn spawn_worker_thread(tls: VMThread, ctx: Option<&GCWorker<OpenJDK>>) {
+    fn spawn_worker_thread(tls: VMThread, ctx: Option<Box<GCWorker<OpenJDK>>>) {
         let ctx_ptr = if let Some(r) = ctx {
-            r as *const GCWorker<OpenJDK> as *mut GCWorker<OpenJDK>
+            Box::into_raw(r)
         } else {
             std::ptr::null_mut()
         };