C++ Armadillo and OpenMp: Parallelization of summation of outer products - define reduction for Armadillo matrix

I am trying to parallelize a for loop using OpenMP which sums over Armadillo matrices. I have the following code:

#include <armadillo>
#include <omp.h>

int main()
{

        arma::mat A = arma::randu<arma::mat>(1000,700);
        arma::mat X = arma::zeros(700,700);
        arma::rowvec point = A.row(0);

        # pragma omp parallel for shared(A) reduction(+:X)
        for(unsigned int i = 0; i < A.n_rows; i++){
                arma::rowvec diff = point - A.row(i);
                X += diff.t() * diff; // Adding the matrices to X here
        }

}

I am getting this error:

[Legendre@localhost ~]$ g++ test2.cpp -o test2 -O2 -larmadillo -fopenmp
test2.cpp: In function ‘int main()’:
test2.cpp:11:52: error: user defined reduction not found for ‘X’

I read up on defining reductions, but I haven't found examples for working with Armadillo matrices. What is the best way to define a reduction for Armadillo matrices in my case?