diff --git a/mmtk/Cargo.lock b/mmtk/Cargo.lock
index 9206cc0..c550d81 100644
--- a/mmtk/Cargo.lock
+++ b/mmtk/Cargo.lock
@@ -425,7 +425,7 @@ dependencies = [
 [[package]]
 name = "mmtk"
 version = "0.24.0"
-source = "git+https://github.com/mmtk/mmtk-core.git?rev=5a01555b5aee984b634f81e9d137f1ae0410fd39#5a01555b5aee984b634f81e9d137f1ae0410fd39"
+source = "git+https://github.com/mmtk/mmtk-core.git?rev=6aa75cd72247d12b9568dc321c46feca6572a512#6aa75cd72247d12b9568dc321c46feca6572a512"
 dependencies = [
  "atomic",
  "atomic-traits",
@@ -460,7 +460,7 @@ dependencies = [
 [[package]]
 name = "mmtk-macros"
 version = "0.24.0"
-source = "git+https://github.com/mmtk/mmtk-core.git?rev=5a01555b5aee984b634f81e9d137f1ae0410fd39#5a01555b5aee984b634f81e9d137f1ae0410fd39"
+source = "git+https://github.com/mmtk/mmtk-core.git?rev=6aa75cd72247d12b9568dc321c46feca6572a512#6aa75cd72247d12b9568dc321c46feca6572a512"
 dependencies = [
  "proc-macro-error",
  "proc-macro2",
diff --git a/mmtk/Cargo.toml b/mmtk/Cargo.toml
index 2484c39..ab68444 100644
--- a/mmtk/Cargo.toml
+++ b/mmtk/Cargo.toml
@@ -37,10 +37,10 @@ features = ["is_mmtk_object", "object_pinning"]
 
 # Uncomment the following lines to use mmtk-core from the official repository.
 git = "https://github.com/mmtk/mmtk-core.git"
-rev = "5a01555b5aee984b634f81e9d137f1ae0410fd39"
+rev = "6aa75cd72247d12b9568dc321c46feca6572a512"
 
 # Uncomment the following line to use mmtk-core from a local repository.
-#path = "../../mmtk-core"
+# path = "../../mmtk-core"
 
 [features]
 default = []
diff --git a/mmtk/src/api.rs b/mmtk/src/api.rs
index 0a8303b..93b593a 100644
--- a/mmtk/src/api.rs
+++ b/mmtk/src/api.rs
@@ -17,6 +17,7 @@ use mmtk::memory_manager;
 use mmtk::memory_manager::mmtk_init;
 use mmtk::util::alloc::AllocatorInfo;
 use mmtk::util::alloc::AllocatorSelector;
+use mmtk::util::apiutils::NullableObjectReference;
 use mmtk::util::constants::MIN_OBJECT_SIZE;
 use mmtk::util::options::GCTriggerSelector;
 use mmtk::util::options::PlanSelector;
@@ -208,10 +209,8 @@ pub extern "C" fn mmtk_is_live_object(object: ObjectReference) -> bool {
 }
 
 #[no_mangle]
-pub extern "C" fn mmtk_get_forwarded_object(object: ObjectReference) -> ObjectReference {
-    object
-        .get_forwarded_object::<Ruby>()
-        .unwrap_or(ObjectReference::NULL)
+pub extern "C" fn mmtk_get_forwarded_object(object: ObjectReference) -> NullableObjectReference {
+    object.get_forwarded_object::<Ruby>().into()
 }
 
 #[no_mangle]
diff --git a/mmtk/src/object_model.rs b/mmtk/src/object_model.rs
index 2ff1e11..e8c581e 100644
--- a/mmtk/src/object_model.rs
+++ b/mmtk/src/object_model.rs
@@ -51,11 +51,13 @@ impl ObjectModel<Ruby> for VMObjectModel {
         let from_start = from_acc.obj_start();
         let object_size = from_acc.object_size();
         let to_start = copy_context.alloc_copy(from, object_size, MIN_OBJ_ALIGN, 0, semantics);
+        debug_assert!(!to_start.is_zero());
         let to_payload = to_start.add(OBJREF_OFFSET);
         unsafe {
             copy_nonoverlapping::<u8>(from_start.to_ptr(), to_start.to_mut_ptr(), object_size);
         }
-        let to_obj = ObjectReference::from_raw_address(to_payload);
+        // unsafe: `to_payload`` cannot be zero because `alloc_copy`` never returns zero.
+        let to_obj = unsafe { ObjectReference::from_raw_address_unchecked(to_payload) };
         copy_context.post_copy(to_obj, object_size, semantics);
         trace!("Copied object from {} to {}", from, to_obj);
 
@@ -123,7 +125,8 @@ impl ObjectModel<Ruby> for VMObjectModel {
     }
 
     fn address_to_ref(addr: Address) -> ObjectReference {
-        ObjectReference::from_raw_address(addr)
+        debug_assert!(!addr.is_zero());
+        unsafe { ObjectReference::from_raw_address_unchecked(addr) }
     }
 
     fn get_size_when_copied(object: ObjectReference) -> usize {
diff --git a/mmtk/src/reference_glue.rs b/mmtk/src/reference_glue.rs
index 5cf8593..1272bd5 100644
--- a/mmtk/src/reference_glue.rs
+++ b/mmtk/src/reference_glue.rs
@@ -8,7 +8,7 @@ pub struct VMReferenceGlue {}
 impl ReferenceGlue<Ruby> for VMReferenceGlue {
     type FinalizableType = ObjectReference;
 
-    fn get_referent(_object: ObjectReference) -> ObjectReference {
+    fn get_referent(_object: ObjectReference) -> Option<ObjectReference> {
         unimplemented!()
     }
 
@@ -19,4 +19,8 @@ impl ReferenceGlue<Ruby> for VMReferenceGlue {
     fn enqueue_references(_references: &[ObjectReference], _tls: VMWorkerThread) {
         unimplemented!()
     }
+
+    fn clear_referent(_new_reference: ObjectReference) {
+        unimplemented!()
+    }
 }