@KGregory I believe your problem here is that you are trying to index into :agcbm, which is not permitted within this function. If you want to update the vector array, you will need to replace the entire array, i.e. update_param!(m, :agcbm, new_array] where the new_array is a 16 element vector as well. Does that make sense for your use case? Of course, your new array might just be a replica of the original one used in set_param! with the first value modified. I assume you can access that original vector from the original set_param! function, or pulling the values out of the already run model?