Error: Number of Equations Exceeds Number of Variables. What is the source of this error in my code?

The following code produces the error mentioned in the subject line:

inputs
  Q = { 0, 'm^3/s' }; % Q:left
  P = { 0, 'Pa' }; %P:left
end

nodes
  I = foundation.hydraulic.hydraulic; % I:left
  O = foundation.hydraulic.hydraulic; % O:right
end

variables
  q = { 0, 'm^3/s' };
  p_i = { 0 , 'Pa' };
  p_o = { 0 , 'Pa' };
end

function setup
  through( q, I.q, O.q );
  across( p_i, I.p, [] );
  across( p_o, O.p, [] );
end

equations
  q == Q;
  p_o == P;
end

Basically I'm trying to use two inputs to control the flow and the pressure out of this block. I do not want the input pressure to be affected though, which is why I separated the across variables as shown in the setup section. I think my problem is a lack of fundamental understanding by what SimScape is looking for when it establishes the implicit relationship in the equation section. This code block was my best attempt at stitching together the ideal pressure source and ideal flow source components. If there is a simpler way to do this, that would be helpful as well.

Any ideas on the source of my error? Thanks for the help.