1 files changed, 1 insertions, 0 deletions
diff --git a/proc.c b/proc.c
index befa8834cd..34159480e0 100644
--- a/proc.c
+++ b/proc.c
@@ -2748,6 +2748,7 @@ Init_Proc(void)
     sysstack_error = rb_exc_new3(rb_eSysStackError,
 				 rb_obj_freeze(rb_str_new2("stack level too deep")));
     OBJ_TAINT(sysstack_error);
+    OBJ_FREEZE(sysstack_error);
 
     /* utility functions */
     rb_define_global_function("proc", rb_block_proc, 0);