diff --git a/gc.c b/gc.c
index 28fa233..aab0963 100644
--- a/gc.c
+++ b/gc.c
@@ -3005,7 +3005,7 @@ rb_objspace_call_finalizer(rb_objspace_t *objspace)
 	while (p < pend) {
 	    if (BUILTIN_TYPE(p) == T_DATA &&
 		DATA_PTR(p) && RANY(p)->as.data.dfree &&
-		RANY(p)->as.basic.klass != rb_cThread && RANY(p)->as.basic.klass != rb_cMutex) {
+		!rb_obj_is_thread((VALUE)p) && !rb_obj_is_mutex((VALUE)p) ) {
 		p->as.free.flags = 0;
 		if (RTYPEDDATA_P(p)) {
 		    RDATA(p)->dfree = RANY(p)->as.typeddata.type->function.dfree;
diff --git a/internal.h b/internal.h
index 4e39e47..2ef1fc4 100644
--- a/internal.h
+++ b/internal.h
@@ -27,6 +27,9 @@ struct rb_classext_struct {
 
 VALUE rb_big_uminus(VALUE x);
 
+VALUE rb_obj_is_thread(VALUE obj);
+VALUE rb_obj_is_mutex(VALUE obj);
+
 #if defined(__cplusplus)
 #if 0
 { /* satisfy cc-mode */
diff --git a/thread.c b/thread.c
index 1614bfe..34f3ad8 100644
--- a/thread.c
+++ b/thread.c
@@ -3350,6 +3350,17 @@ static const rb_data_type_t mutex_data_type = {
     {mutex_mark, mutex_free, mutex_memsize,},
 };
 
+VALUE
+rb_obj_is_mutex(VALUE obj)
+{
+    if (rb_typeddata_is_kind_of(obj, &mutex_data_type)) {
+	return Qtrue;
+    }
+    else {
+	return Qfalse;
+    }
+}
+
 static VALUE
 mutex_alloc(VALUE klass)
 {
diff --git a/vm.c b/vm.c
index bddb4dd..57cb090 100644
--- a/vm.c
+++ b/vm.c
@@ -1780,6 +1780,17 @@ const rb_data_type_t ruby_thread_data_type = {
     },
 };
 
+VALUE
+rb_obj_is_thread(VALUE obj)
+{
+    if (rb_typeddata_is_kind_of(obj, &thread_data_type)) {
+	return Qtrue;
+    }
+    else {
+	return Qfalse;
+    }
+}
+
 static VALUE
 thread_alloc(VALUE klass)
 {