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?