Struct contents reference from a non-struct array object error

Hi,

In the following code, the mentioned error is announced after a iteration. What is the reason for this that there is no error in the first iteration and why it is wrong in the second iteration

error: Struct contents reference from a non-struct array object

f=[pop.cost];

f=1./f;

f=f./sum(f);

f=cumsum(f);

%Crossover

pop2=repmat(empty_individual,Nc/2,2);

for i=1:Nc/2

i1=find(rand<=f,1,'first');

i2=find(rand<=f,1,'first');

%Select parent

P1=(pop(i1));

P2=(pop(i2));

Ch_1.beta=(R1.*P1.beta)+(R2.*P2.beta);

Ch_2.beta=(R2.*P1.beta)+(R1.*P2.beta);

Ch_1.sigma=(R1.*P1.sigma)+(R2.*P2.sigma);

Ch_2.sigma=(R2.*P1.sigma)+(R1.*P2.sigma);

Ch_1.ro=(R1.*P1.ro)+(R2.*P2.ro);

Ch_2.ro=(R2.*P1.ro)+(R1.*P2.ro);