Error: "Attempt to evaluate negative power of zero."

I'm building a model using the Reaching Flow in Porous Media in a 2D axis-symmetric component, and my goal is to remove some of the dilute species from the system as a function of concentration and fluid velocity (e.g. the reaction slows as concentration decreases, and is increased when fluid velocity is high).

I can get the model to behave as expected, but I cannot successfully implement a velocity dependent Reaction. I am able to use a constant (e.g. -0.01 [mol/m^3/s]) or concentration dependent reaction rate, but if I try to use average velocity I get the error:

Attempt to evaluate negative power of zero.
- Function: ^
Failed to evaluate temporary symbolic derivative variable.
- Variable: comp1.rfds.U@VDN$comp1.w2
- Defined as: (0.5*(((comp1.u2^2)+(comp1.w2^2))^(-0.5)))*(2*comp1.w2)
Failed to evaluate expression.
- Expression: d(comp1.rfds.U,comp1.w2)

I attempted to incorporate the velocity in the porous domain (or any domain.. just trying to get anything to work for starters) by setting up an Average of the porous domain (Component --> Definitions --> Average), then defining a variable as the velocity magnitude of that domain (comp1.aveop1(rfds.U)).

I have consulted other threads here and saw recommendations to use 'eps' when defining my variable to prevent my initial expression from being zero, but this doesn't seem to help. Any guidance is appreciated, and I can provide further details of the model as necessary.