Question with 'at' operator

Hi guys,

When using Darcy flow module, I want to use a pressure dependent permeability.

Let's say k = k_0*(p/p(at time zero)), and I used 'at' operator to get the pressure at time zero as 'at(1,p)'.

However, it comes to an error saying "The at() operator cannot be used in this context. Failed to evaluate expression.Expression: at(1,mod1.p)"

Could you please give me some clue what's the right way to use the solution at a specific time point?

Regards