Failing attempt to get shell command output in Conkeror

shell_command_output.fail.js

function shell_command_output (command) {
    var error_text = "";
    var output_text = "";
    var exit_status = shell_command(
        command,
        $fds = [
            {
                output:
                async_binary_string_writer("")
            },
            {
                input:
                async_binary_reader(
                    function (s) {
                        output_text += s || "";
                    }
                )
            },
            {
                input:
                async_binary_reader(
                    function (s) {
                        error_text += s || "";
                    }
                )
            }
        ]
    );
    if (exit_status != 0 ) {
        throw new Error("exit_status: " + exit_status + ", error_text: " +
                        error_text);
    }
    return output_text.trim();
}