"pointer being freed was not allocated" while creating an array of template instantiations in C++

Question

I will simplify the problem in order to present a minimal and reproducible example:

I have a class Polynomial which is just a wrapper around an array of coefficients. For convenience in my analyses downstream it is a good idea to write it as a template regarding to the max. degree it can represent. Please remember that an N-th degree polynomial will hold N+1 coefficients.

template <const unsigned int MaxDeg>
class Polynomial {

  public:

    int *coefficients;

    Polynomial(int* arr) {
        coefficients = new int[MaxDeg+1];
        for (unsigned int i=0; i <= MaxDeg; i++) coefficients[i] = arr[i];
    }

    Polynomial() {
        coefficients = new int[MaxDeg+1]();
    }

    ~Polynomial() {
        delete[] coefficients;
    }

    int& operator[](int index) const {
        return coefficients[index];  
    }
};

In my main I test the code with creating one polynomial and then a one-sized array for it:

    int f_q[] = {5, 9, 6, 16, 4, 15, 16, 22, 20, 18, 30};
    Polynomial<10> P_f_q(f_q);
    Polynomial<10> P_arr[1] = {P_f_q}; // this line raises error

I get the following memory error:

test(33770,0x10de6be00) malloc: *** error for object 0x7fef43605420: pointer being freed was not allocated
test(33770,0x10de6be00) malloc: *** set a breakpoint in malloc_error_break to debug
[1]    33770 abort      ./test

In reality my code is much more complex but I commented out as much as I could and I really can't see where does the error come from(?)