Bench Update Error : Site not ready to migrate to version 4

I get this error when I run bench update

Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/girish/progress/frappe-bench/apps/frappe/frappe/utils/bench_helper.py", line 79, in <module>
    main()
  File "/home/girish/progress/frappe-bench/apps/frappe/frappe/utils/bench_helper.py", line 16, in main
    click.Group(commands=commands)(prog_name='bench')
  File "/home/girish/progress/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/home/girish/progress/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/home/girish/progress/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/girish/progress/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/girish/progress/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/girish/progress/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/girish/progress/frappe-bench/env/local/lib/python2.7/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/girish/progress/frappe-bench/apps/frappe/frappe/commands/__init__.py", line 24, in _func
    ret = f(frappe._dict(ctx.obj), *args, **kwargs)
  File "/home/girish/progress/frappe-bench/apps/frappe/frappe/commands/site.py", line 209, in migrate
    migrate(context.verbose, rebuild_website=rebuild_website)
  File "/home/girish/progress/frappe-bench/apps/frappe/frappe/migrate.py", line 30, in migrate
    frappe.modules.patch_handler.run_all()
  File "/home/girish/progress/frappe-bench/apps/frappe/frappe/modules/patch_handler.py", line 29, in run_all
    if not run_single(patchmodule = patch):
  File "/home/girish/progress/frappe-bench/apps/frappe/frappe/modules/patch_handler.py", line 63, in run_single
    return execute_patch(patchmodule, method, methodargs)
  File "/home/girish/progress/frappe-bench/apps/frappe/frappe/modules/patch_handler.py", line 83, in execute_patch
    frappe.get_attr(patchmodule.split()[0] + ".execute")()
  File "/home/girish/progress/frappe-bench/apps/erpnext/erpnext/patches/v4_0/validate_v3_patch.py", line 11, in execute
    raise Exception, "site not ready to migrate to version 4"
Exception: site not ready to migrate to version 4

Please help me resolve this issue. Thanks in advance.

Check the code in line 11 in the validate_v3_patch.py file, maybe it may give you some idea about the error