diff --git a/src/duk_builtin_thread.c b/src/duk_builtin_thread.c
index fa293821..3dbabc7f 100644
--- a/src/duk_builtin_thread.c
+++ b/src/duk_builtin_thread.c
@@ -285,15 +285,3 @@ int duk_builtin_thread_current(duk_context *ctx) {
 	return 1;
 }
 
-/*
- *  toString(), valueOf()
- */
-
-int duk_builtin_thread_prototype_to_string(duk_context *ctx) {
-	return DUK_RET_UNIMPLEMENTED_ERROR;
-}
-
-int duk_builtin_thread_prototype_value_of(duk_context *ctx) {
-	return DUK_RET_UNIMPLEMENTED_ERROR;
-}
-
diff --git a/src/duk_heap_alloc.c b/src/duk_heap_alloc.c
index e5e594fd..1bf231c3 100644
--- a/src/duk_heap_alloc.c
+++ b/src/duk_heap_alloc.c
@@ -285,7 +285,7 @@ static int init_heap_thread(duk_heap *heap) {
 	thr = duk_hthread_alloc(heap,
 	                        DUK_HOBJECT_FLAG_EXTENSIBLE |
 	                        DUK_HOBJECT_FLAG_THREAD |
-	                        DUK_HOBJECT_CLASS_AS_FLAGS(DUK_HOBJECT_CLASS_OBJECT));
+	                        DUK_HOBJECT_CLASS_AS_FLAGS(DUK_HOBJECT_CLASS_THREAD));
 	if (!thr) {
 		DUK_DPRINT("failed to alloc heap_thread");
 		return 0;
@@ -305,6 +305,9 @@ static int init_heap_thread(duk_heap *heap) {
 	/* FIXME: this may now fail, and is not handled correctly */
 	duk_hthread_create_builtin_objects(thr);
 
+	/* default prototype (Note: 'thr' must be reachable) */
+	DUK_HOBJECT_SET_PROTOTYPE(thr, (duk_hobject *) thr, thr->builtins[DUK_BIDX_THREAD_PROTOTYPE]);
+
 	return 1;
 }
 
diff --git a/src/genbuiltins.py b/src/genbuiltins.py
index 799abb95..cea679e9 100644
--- a/src/genbuiltins.py
+++ b/src/genbuiltins.py
@@ -1074,8 +1074,7 @@ bi_thread_prototype = {
 	'values': [
 	],
 	'functions': [
-		{ 'name': 'toString',			'native': 'duk_builtin_thread_prototype_to_string',	'length': 0 },
-		{ 'name': 'valueOf',			'native': 'duk_builtin_thread_prototype_value_of',	'length': 0 },
+		# toString() and valueOf() are inherited from Object.prototype
 	],
 }