You have two options.
One, like I mentioned earlier, is to manually type the variable based on what you can observe coming out of the iterator. This saves operations.
Two, you can do a set variable with built-in functions, and then only map the array variable in the iterator. Sure it costs one more operation, but as long as you don’t use built-in functions directly in the iterator, the iterator module should output mappable variables. This fixes the cause.