Failed to evaluate variable: freq

Hi COMSOL Users,

I'm new to using the v4.x series of COMSOL, so perhaps I've missed something simple, but I get an error when trying to plot body volume force variables. The error message states:
"Cannot evaluate expression.
Failed to evaluate variable.
- Variable: freq
- Geometry: 1
- Domain: 1
Failed to evaluate variable.
- Variable: mod1.acpr.freq
- Defined as: freq
..."
and continues down to mod1.spf.Fx which I am trying to plot (see attached image).

I have used the Pressure Acoustics module along with the Laminar Flow module. I set the solver to first solve Pressure Acoustics in the frequency domain (Step 1), then a combination of parameters from Pressure Acoustics is used to calculate body forces in the Laminar Flow module. Stationary analysis is used to solve for the Laminar Flow physics (Step 2).

Has anyone had this problem before? Is there an easy way to fix this problem?

Thanks in advance,
Ian