Calculating a series function

Dear Matlab Community,

i have a problem regarding the following equation which i want to calculate: The Input are lambda, beta, D_z, M_inf, h and K

The output should be M(t) = M_t:

My Code is as follows but it is not working properly:

M_inf = 1.85;
mu = 0.1;
lambda = 4.51;
beta = 4.51;
D_z = 5.32;
h = 1.5;
K = pi^2*D_z/h^2;

M_t=zeros(600,1);
for t=1:600
        syms i 
        r_plus = 0.5*((K*i^2+lambda+beta)+sqrt((K*i^2+lambda+beta)^2-4*K*beta*i^2));
        r_minus = 0.5*((K*i^2+lambda+beta)-sqrt((K*i^2+lambda+beta)^2-4*K*beta*i^2)); 
        
        
        
        M_t(t,1) =M_inf*(1-8/(pi^2)*symsum((r_plus*exp(-r_minus*t)-r_minus*exp(-r_plus*t))/(i^2*(r_plus-r_minus)),i,1,inf)...
            +8/(pi^2)*(K*(beta/(beta+lambda)))*symsum((exp(-r_minus*t)-exp(-r_plus*t))/(r_plus-r_minus),i,1,inf));
end

I hope somebody can help me