diff --git a/yjit_iface.c b/yjit_iface.c
index 6e72a2cc91..9240e3f1c9 100644
--- a/yjit_iface.c
+++ b/yjit_iface.c
@@ -1277,6 +1277,7 @@ rb_yjit_init(struct rb_yjit_options *options)
 
     // YJIT::Block (block version, code block)
     cYjitBlock = rb_define_class_under(mYjit, "Block", rb_cObject);
+    rb_undef_alloc_func(cYjitBlock);
     rb_define_method(cYjitBlock, "address", block_address, 0);
     rb_define_method(cYjitBlock, "id", block_id, 0);
     rb_define_method(cYjitBlock, "code", block_code, 0);