DEFINE_SOURCE error?
 

I'm trying to implement the RAS turbulence model and am having a problem with the R transport equation. If I hard code a value for R and turn the equation off I get reasonable results and trends for all my variables. When I turn the R equation on the solution diverges or I get non physical results. Based on trial and error, I think the problem is in my DEFINE_SOURCE.

DEFINE_SOURCE(r_source, c, t, dS, eqn)
{
real source;
source = C_UDMI(c,t,C1)*C_R(c,t)*sqrt(C_UDMI(c,t,P)*C_UDSI( c,t,RTilda))-C_UDMI(c,t,C2)*C_R(c,t)*SQR(C_UDSI_G(c,t,RTilda)[0]+C_UDSI_G(c,t,RTilda)[1]);
dS[eqn] = 0.0;
return source;
}

RTilda is a UDS so that I can use C_UDSI_G to get the gradient easily. Is my use of C_UDSI_G correct? Is there some other error here?