Cannot assign void to an implicitly-typed variable for WaitUntilAvailable() extension method

i want submit reactjs web site two step form.
i wrote a code like you'r sample for test :

            var configuration = Configuration.Default
                .WithDefaultLoader()
                .WithDefaultCookies()
                .WithJs();
        var context = BrowsingContext.New(configuration);
            var queryDocument =  await context.OpenAsync("https://www.site.com/_/login?").WaitUntilAvailable();

but give me an error

Cannot assign void to an implicitly-typed variable

sample code :

var context = BrowsingContext.New(config);
var document = await context.OpenAsync(address)
    .WaitUntilAvailable();

why give me an error ?

[FlorianRappl]

Landed in devel.