Sqlite Update query with SqliteModernCpp

I am using the SqliteModernCpp library. I have a data access object pattern, including the following function:

void movie_data_access_object::update_movie(movie to_update)
{
    // connect to the database
    sqlite::database db(this->connection_string);

    // execute the query
    std::string query = "UPDATE movies SET title = " + to_update.get_title() + " WHERE rowid = " + std::to_string(to_update.get_id());
    db << query;
}

Essentially, I want to update the record in the database whose rowid (the PK) has the value that the object to_update has in its parameter (which is returned by get_id()).

This code yields an SQL logic error. What is the cause of this?