diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 2c36fee..9ac0bb5 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -67,7 +67,7 @@ jobs:
           if [[ ${{ matrix.gc.mmtk_build }} == debug ]]; then
             echo 'RUST_LOG=' >> $GITHUB_ENV
             echo 'RUBY_TEST_TIMEOUT_SCALE=20' >> $GITHUB_ENV
-            echo 'SYNTAX_SUGGEST_TIMEOUT=60' >> $GITHUB_ENV
+            echo 'SYNTAX_SUGGEST_TIMEOUT=120' >> $GITHUB_ENV
           fi
           echo "EXCLUDES=$PWD/test/.excludes-mmtk" >> $GITHUB_ENV
           echo "MSPECOPT=-B$PWD/spec/mmtk.mspec" >> $GITHUB_ENV
diff --git a/gc/mmtk/extconf.rb b/gc/mmtk/extconf.rb
index 5f42289..c0e7880 100644
--- a/gc/mmtk/extconf.rb
+++ b/gc/mmtk/extconf.rb
@@ -15,6 +15,10 @@
     MMTK_BUILD = debug
     LIBMMTK_RUBY = libmmtk_ruby.#$LIBEXT
     RUSTSRCS = #{rustsrcs.join(" \\\n\t   ")}
+
+    ifeq ($(MMTK_BUILD), debug)
+    CPPFLAGS += -DMMTK_DEBUG
+    endif
     MAKEFILE
   ]
 end
diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index 042517e..38e730a 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -963,6 +963,18 @@ rb_gc_impl_writebarrier(void *objspace_ptr, VALUE a, VALUE b)
 
     if (SPECIAL_CONST_P(b)) return;
 
+#ifdef MMTK_DEBUG
+    if (!rb_gc_impl_pointer_to_heap_p(objspace_ptr, (void *)a)) {
+        char buff[256];
+        rb_bug("a: %s is not an object", rb_raw_obj_info(buff, 256, a));
+    }
+
+    if (!rb_gc_impl_pointer_to_heap_p(objspace_ptr, (void *)b)) {
+        char buff[256];
+        rb_bug("b: %s is not an object", rb_raw_obj_info(buff, 256, b));
+    }
+#endif
+
     mmtk_object_reference_write_post(cache->mutator, (MMTk_ObjectReference)a);
 }