Question: Inexplicable Error while solving for Steady State Solutions

I was trying to solve for the equilibrium points for a system of differential equations.  Solving for the last equation, I encountered an error that I can't seem to find anywhere else.  "Error, (in Engine:-Dispatch) not implemented yet: 13". The code is below if anyone wants to take a go a figuring what's up.  Thanks!

A:=Asource/muA;
                            Asource
                            -------
                              muA  
dTRdt:=aR*qR*A-muR*TR+rR*TR*TE;
               aR qR Asource                    
               ------------- - muR TR + rR TR TE
                    muA                         

TR_SS:=solve(dTRdt=0,TR);
                          aR qR Asource   
                      - ------------------
                        muA (-muR + rR TE)
DTEdt:=aE*qE*A/(alpha+TR_SS)-muE*TE+rE*TE(1-TE/TEmax);
           aE qE Asource                           /     TE  \
  -------------------------------- - muE TE + rE TE|1 - -----|
      /          aR qR Asource   \                 \    TEmax/
  muA |alpha - ------------------|                            
      \        muA (-muR + rR TE)/                            
TE_SS:=solve(DTEdt=0,TE);
Error, (in Engine:-Dispatch) not implemented yet: 13