MPSC Queue Design Issue (thread cannot join)

Question

I have the following MpscQueue implementation

EDIT: added an is_running atomic, but problem still persists.

template<typename T>
class MpscQueue {
    public:
    MpscQueue() = default;
    MpscQueue(MpscQueue&&) = delete;

    bool wait_and_pop(T& val, std::atomic<bool>& is_running) {
        std::unique_lock<std::mutex> lock(mutex);

        cond_var.wait(lock,
            [this, &is_running]{ return queue.size() > 0 || !is_running; });

        if (!is_running) return false;

        val = std::move(queue.front());
        queue.pop();
        return true;
    }

    template<typename U>
    void push(U&& val) {
        auto const is_empty = [&]{
            auto const lock = std::unique_lock(mutex);
            auto const res = queue.empty();

            queue.push(std::forward<U>(val));

            return res;
        }();

        if (is_empty) cond_var.notify_one();
    }

    private:
    std::queue<T> queue;
    std::mutex mutex;
    std::condition_variable cond_var;
};

I am attempting to pop a value like this

// At some point earlier
MpscQueue<Message> mailbox;
std::atomic<boo> is_running{true}; // Is set to false at a later time
void run_once() {
    Message m;
    mailbox.wait_and_pop(m, is_running);
    // process_message(std::move(m));
}

The above code run_once is being fed into the thread constructor. My issue is that if I attempt to join the thread that this is on, it gets stuck in the condition variable wait condition. What would be the best way to solve this? I tried passing an atomic by reference as a parameter into wait_and_pop but it did not seem to be updating and also did not seem like a smart implementation decision.