Assignment has more non-singleton rhs dimensions than non-singleton subscripts

Hello, I am trying to take expression values of 570 genes (let's call it a) from 13514 gene cell array (lets call it b). B involves most of the genes in a but when it cannot find the string in b, it gives me the error message "Assignment has more non-singleton rhs dimensions than non-singleton subscripts". My code is:

exprs_cont = zeros(length(modelgenes),1);
for i=1:length(modelgenes);
    a(i,1) = find(strcmp(genes,modelgenes(i)));
    exprs_cont(i,1)=mean_cnt(a,1); 
end

Actually, I want this to give -1 when cannot find any matching string. Anyone have advices? (It's been about 35 hours without sleep so sorry for my grammar mistakes)

[Birdman]

Can you share your variables as well?

[Hamza Karakurt]

My variables are here.

Thank you.

[Birdman]

mean_cnt?