ValueError when sending two emails with the same template

[IsNeron]

I've been trying to send two messages with the same body. The code for it is

class MailService:
    def __init__(self, background_tasks: BackgroundTasks) -> None:
        self._background_tasks = background_tasks

    def _send(
        self,
        subject: str,
        recipients: 'list[EmailStr]',
        template: str,
        template_body: dict,
    ) -> None:
        support_email = settings.mail.support_email
        template_body.update(
            app_title=settings.title,
            support_email=support_email,
        )
        self._background_tasks.add_task(mail_sender.send_message,
            MessageSchema(
                subject=f'{settings.title} - {subject}',
                recipients=recipients,
                subtype=MessageType.html,
                reply_to=[settings.mail.support_email],
                template_body=template_body,
            ),
            template,
        )

    def invite_unregistered_to_members(
        self,
        sender_name: str,
        entity_table: 'type[orm.IdNameMixin]',
        entity_name: str,
        receiver_email: 'EmailStr',
    ) -> None:
        public_url = settings.public_url
        self._send(
            'приглашение',
            [receiver_email],
            'invite_unregistered_to_members.jinja2',
            {
                'entity_kind': entity_table.__tablename__,  # type: ignore
                'entity_name': entity_name,
                'sender_name': sender_name,
                'public_url': public_url,
                'login_link': settings.template.login_link,
            },
        )

When I'm trying to call invite_unregistered_to_members, the first sent works OK, but the second letter to another e-mail fails with the ValueError: Unable to build template data dictionary - <class 'str'>is an invalid source data type

I suppose, that the error is caused by Fastmail.py -- def check_data

    @staticmethod
    def check_data(data: Union[Dict[Any, Any], str, None]) -> Dict[Any, Any]:
        if not isinstance(data, dict):
            raise ValueError(
                f"Unable to build template data dictionary - {type(data)}"
                "is an invalid source data type"
            )

        return data

In data there is somehow a jinja2 template...

The only fix that i found is to restart or reload app, then letter will send fine. But another letter after it will fail again.

Can anybody help with this? Please...

[DurandA]

I got the same issue.

[cosmofactory]

Same issue here, any solutions?