Question: Passing Functions to a Matrix

In Maple 18, I'm trying to define a matrix consisting of the output of the diff function from Physics, but it seems to return an empty matrix for some reason.

First, the setup:

with(Physics);
(D(x))(t) := x(t)^2+y(t)^2;
(D(y))(t) := x(t)^2-y(t)^2;


Then if I try to input the output of e.g. diff(x'(t),x(t)) directly into a matrix structure, it outputs an empty space for that entry.

I can work around it setting the output of diff into variables first, and then using those in my matrix structure, but that's a bit annoying.

See attached image for input/output

 

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