differential equations – Replace in a Symbolic Derivative doesnt work with Pi/2

I was writing some small functions for GR applications, and I was defining a Function that gives me the Geodesic equations. When testing if those worked with the Schwarzschild Metric I came upon a problem when trying to replace the angle $theta$ with $frac{pi}{2}$ this then didn’t properly simplify it when there are derivatives of $theta$.

I have made a simple example to showcase what my problem is:

Sum(D(xx((i))((Tau)), (Tau)), {i, 4}) /. t -> Pi/2

This produces the following output:

$left(frac{pi }{2}right)'(tau )+x'(tau )+y'(tau )+z'(tau )$

What can I do to either prevent this from happening or resolve this issue?