Question: sum, diff and subs

I am sorry. There is no 'tag' button on my computer, so I can't ask question here. I don't know the reason.

So I ask here. Now, my real question is:

sum(subs(x = 0, diff(x^2, x $ k)), k = 1 .. 2);


subs(x = 0, diff(x^2, x $ 2));

I think the first result should be 2, but it gives me 0.

Thanks in advance.


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