Error: Subscripted assignment dimension mismatch

function nn_params = unroll_parameters(parameters)
nn_params = [];
for i = 1: length(parameters)/2
    WI = parameters(strcat('W', num2str(i)));
    WI_unrolled = (WI(:));
    nn_params(:,end+1) = WI_unrolled;
    bI = parameters(strcat('b', num2str(i)));
    bI_unrolled = (bI(:));
    nn_params(:,end+1) = bI_unrolled;
end

I wrote this function to unroll all elements of every keys in a MATLAB map and stor it in an array nn_params. When I use breakpoint within the code, I noticed that whenever it want to append the unrolled, bI_unrolled, it gives the error: Subscripted assignment dimension mismatch.

NB: parameters is a mapp that can contain: W1,b1,W2,b2,...