Call to deleted constructor of std::__1::ostream error on operator<< overload

I am trying to get a function to be recognized as a friend but I can not figure out what I am doing wrong. I tried changing things to references and moving things in and out of the namespace and header cpp. There is nothing that I notice as the culprit for why this does not work.

The error I am getting is

error: call to deleted constructor of 'std::__1::ostream' (aka 'basic_ostream<char>')
          return output;
                 ^~~~~~

I'm thinking it might be a reference issue, or maybe something I forgot to import when I started. But I can't think of anything.

namespace N 
{
    class Complex
    {
    public:
        // Contructors
        Complex(double n, double i)
        {
            this->number = n;
            this->imaginary = i;
        }

        Complex()
        {
            this->number = 0;
            this->number = 0;
        }

        // Accessors
        friend ostream operator<< (ostream& os, const Complex& a);

        // Operator Overloads
        Complex operator + (Complex b)
        {
            Complex c;
            c.number = this->number + b.number;
            c.imaginary = this->imaginary + b.imaginary;
            return c;
        }

    private:
        double number;
        double imaginary;
    };

    ostream operator<<(ostream& output, const Complex& a) 
    {
        output << fixed << setprecision(1) << a.number << " + " << a.imaginary << "i";
        return output;
    }
}