diff --git a/ext/objspace/objspace_dump.c b/ext/objspace/objspace_dump.c
index bcc258eb25..5e2b417fbe 100644
--- a/ext/objspace/objspace_dump.c
+++ b/ext/objspace/objspace_dump.c
@@ -747,6 +747,8 @@ shape_i(rb_shape_t *shape, void *data)
         break;
       case SHAPE_INITIAL_CAPACITY:
         dump_append(dc, "\"INITIAL_CAPACITY\"");
+        dump_append(dc, ", \"capacity\":");
+        dump_append_sizet(dc, shape->capacity);
         break;
       case SHAPE_T_OBJECT:
         dump_append(dc, "\"T_OBJECT\"");