C++ std::shared_from_this() causes std::bad_weak_ptr after the object was constructed with an existing shared_ptr

I am creating a C++ server application using standalone Asio and C++11 and am getting an error, which is why I am asking for help.

The error

In the class worker_thread, during the call to shared_from_this(), a bad_weak_ptr exception is raised, which causes the program to crash.

The layout

1. The class connection_manager creates and stores objects of type std::shared_ptr<worker_thread> inside a std::vector container
2. The class worker_thread inherits from std::enable_shared_from_this<worker_thread>.
3. The class worker_thread creates objects of type std::shared_ptr<connection>.
4. The class connection requires a pointer (which is a shared pointer) to the class worker_thread, so that in can call the void handle_finish(std::shared_ptr<connection>)

Program flow

1. The class worker_thread is created via its constructor, from the class connection_manager using std::make_shared<worker_thread> with two shared pointers as parameters.
2. void init() is called from worker_thread by connection_manager
3. Later in the program, connection_manager calls std::shared_ptr<connection> get_available_connection() from worker_thread
4. During this method's execution, a new connection is created via std::make_shared<connection>, and one of the arguments is the shared pointer to the current worker_thread obtained via shared_from_this()
5. During the shared_from_this() call, the program crashes with a bad_weak_ptr exception.

Research

From my research, the most common causes of this error are:

1. When shared_from_this() is called within a constructor (or a function which is called by the constructor)
2. When there is no existing std::shared_ptr pointing to the object.

In my program:

1. The call to the constructor and the get_available_connection() are separate, and through outputing lines in the terminal, it seems that the worker_thread is constructed and initialised by the time the call to get_available_connection() occurs
2. The connection_manager class holds a shared pointer to every worker_thread object.

Code

All something_ptr are std::shared_ptr<something>

Header files

connection_manager.hpp

typedef asio::executor_work_guard<asio::io_context::executor_type>
    io_context_work;
std::vector<worker_thread_ptr> workers;
std::vector<io_context_ptr> io_contexts;
std::vector<io_context_work> work;

worker_thread.hpp

class worker_thread : std::enable_shared_from_this<worker_thread> {
public:

/// Create a worker thread.
explicit worker_thread(io_context_ptr io, config_ptr vars_global);

void init();
void join();

connection_ptr get_available_connection();
//...

connection.hpp

explicit connection(std::shared_ptr<worker_thread> worker,
            std::shared_ptr<asio::io_context> io, 
            config_ptr vars_parent);

Source files

connection_manager.cpp

connection_manager::connection_manager(config_ptr vars) {
    std::size_t number_of_threads = vars->worker_threads;
    while(number_of_threads > 0) {
        io_context_ptr io_context(new asio::io_context);
        io_contexts.push_back(io_context);
        work.push_back(asio::make_work_guard(*io_context));

        worker_thread_ptr worker =
            std::make_shared<worker_thread>(io_context, vars);
        workers.push_back(worker);
        std::cout << worker.get() << std::endl;

        worker->init();

        --number_of_threads;
    }   
} 

connection_ptr connection_manager::get_available_connection() {
    std::size_t index_of_min_thread = 0;
    std::size_t worker_count = workers.size();
    for(std::size_t i = 1; i < worker_count; ++i) {
        if(workers[i]->active_connection_count() <
                workers[index_of_min_thread]->active_connection_count())
            index_of_min_thread = i;
    }
    return workers[index_of_min_thread]->get_available_connection();
}

worker_thread.cpp

worker_thread::worker_thread(io_context_ptr io, 
        config_ptr vars_global)
    :io_context(io), active_conn_count(0), vars(vars_global),
    worker(
        [this]() {
            if(io_context)
                io_context->run();
        }   
    ) {}

void worker_thread::init() {
    //Additional initialisation, this is called by connection_manager
    //after this thread's construction
}   

connection_ptr worker_thread::get_available_connection() {
    connection_ptr conn;
    if(!available_connections.empty()) {
        conn = available_connections.front();
        available_connections.pop();
        active_connections.insert(conn);
        return conn;
    } else {
        conn = std::make_shared<connection>(shared_from_this(), io_context, vars);
        active_connections.insert(conn);
        return conn;
    }
}

I am sorry if this question has been answered before, but I tried to resolve this, and after trying for some time, I decided it would be better to ask for help.