Yangtools fails to parse module with yin-element statement before vendor extension

Yangtools is failing to parse modules where there's a "yin-element true" statement between an "argument" statement and a vendor extension relating to that augment.

So:

extension foo {
argument text {
vendor:bar

{ type string; }
}
}

is ok. But:

extension foo {
argument text {
yin-element true;
vendor:bar { type string; }

}
}

is not. If I move the yin-element statement below the vendor extension it parses ok.

The logs I get look like:

"extraneous input 'description' expecting

{'}

', 'uses', 'typedef', 'rpc', 'notification', 'list', 'leaf-list', 'leaf', 'identity', 'grouping', 'feature', 'deviation', 'extension', 'container', 'choice', 'augment', 'anyxml', IDENTIFIER} in module..."

The vendor extension has a "description" in it. If I delete that this parses ok.