Array rank reduction... why?

Hi everyone,
I don’t get the “Warning: Converting an array to var would cause array rank reduction and is not permitted” …
Assuming that the definition works anyway, I don’t get if could be an issue on some model or not… I tested the definition on couple of demos and seems fine… Anyone could tell me what’s about?
I know the topic is already been “asked”, but I saw it’s related to the single issue, I didn’t get anyway…

Here’s a screen shot…

the element is connecting a list of “family types” instances…

Thanks!