When revision statement enclose a custom statement, Yang Parser fails to parse correctly

IETF allows custom statement enclosed in the revision statement, as the following YANG ABNF Grammar:

revision-stmt = revision-keyword sep date-expr-str optsep
(";" /
"

{" stmtsep [description-stmt stmtsep] "}

")

stmtsep = *(WSP / line-break / unknown-statement)

unknown-statement = prefix ":" identifier [sep string] optsep
(";" / "

{" *unknown-statement "}

")

However, Yang Parser fails to parse the following:

module "bug-ext" {
yang-version 1;
namespace "urn:test:bug:bug-ext";
prefix "bug-ext";

revision 2012-04-16 {
}

extension migration

{ argument level; }

}

module bug {
yang-version 1;
namespace "urn:test:bug:bug";
prefix "bug";

import bug-ext

{ prefix bug-ext; }

import ietf-inet-types

{ prefix "inet"; }

revision 2012-04-16

{ description "Initial revision"; bug-ext:migration 1; }

leaf test

{ type string; }

}