[Bug #19833] Fix index underflow at superclasses of `BasicObject`

1 files changed, 4 insertions, 0 deletions

diff --git a/object.c b/object.c
index 823b8df2cf..45a571adac 100644
--- a/object.c
+++ b/object.c
@@ -2165,6 +2165,10 @@ rb_class_superclass(VALUE klass)
         if (klass == rb_cBasicObject) return Qnil;
         rb_raise(rb_eTypeError, "uninitialized class");
     }
+
+    if (!RCLASS_SUPERCLASS_DEPTH(klass)) {
+        return Qnil;
+    }
     else {
         super = RCLASS_SUPERCLASSES(klass)[RCLASS_SUPERCLASS_DEPTH(klass) - 1];
         RUBY_ASSERT(RB_TYPE_P(klass, T_CLASS));