How to set field for a moved-from object?

My code snipper is:

my_string& operator=(my_string&& other) 
{
    if (this != &other)
    {
        if (buffer_ != nullptr) 
        {
            delete[] buffer_;
        }

        size_ = std::move(other.size_);
        buffer_ = std::move(other.buffer_);
        //other.size_ = 0; // how to set a value?
    }

    return *this;
}

bool empty() const
{
    return size_ == 0;
}

I want to set size_ for a moved-from object to empty to work correctly.

If I uncomment line other.size_t = 0;, then some internal errors happens.