diff --git a/shape.c b/shape.c
index c0524bd81c..03d3b94ae6 100644
--- a/shape.c
+++ b/shape.c
@@ -7,6 +7,10 @@
 #include "internal/variable.h"
 #include <stdbool.h>
 
+#ifndef SHAPE_DEBUG
+#define SHAPE_DEBUG (VM_CHECK_MODE > 0)
+#endif
+
 static ID id_frozen;
 static ID id_t_object;
 static ID size_pool_edge_names[SIZE_POOL_COUNT];
@@ -339,7 +343,7 @@ rb_shape_rebuild_shape(rb_shape_t * initial_shape, rb_shape_t * dest_shape)
     return midway_shape;
 }
 
-#if VM_CHECK_MODE > 0
+#if SHAPE_DEBUG
 VALUE rb_cShape;
 
 /*
@@ -617,7 +621,7 @@ Init_default_shapes(void)
 void
 Init_shape(void)
 {
-#if VM_CHECK_MODE > 0
+#if SHAPE_DEBUG
     rb_cShape = rb_define_class_under(rb_cRubyVM, "Shape", rb_cObject);
     rb_undef_alloc_func(rb_cShape);