How do I assign a complex value into a non-complex location?

I have the following code that will calculate the rotation of a motor based upon an input flux and temperature. The function is being run in Simulink, but I keep getting the error "Cannot assign a complex value into a non-complex location" at the last line in the function (that actually calculates Theta). I can see that theta may be complex due to the feedback loop that was built around the temperature T, but I don't see how to solve it. I have attached a picture of the simulink model as well.

Thanks,

Greg

function Theta  = flow_rate(Q,T)
%Calc theta position
Theta=complex(0);
Ti=600; %constant inlet particle temperature (degree C)

%Convert the output of the servo motor (theta) to linear distance, assumes
%3000 counts per turn 
slot_length=1.16;% m
%Heat Transfer Model
Cp=1200; %specific heat of the particles
%beverloo Constants and parameters
C1=19;
C2=8.9;
rho_b=2000; %bulk density, kg/m^3
d_p=0.35e-3;%particle diameter, 350 micron
g=9.81; %gravity
Theta=((Q/(C1*rho_b*sqrt(g)*slot_length*Cp*(T-Ti)))^(2/3)+C2*d_p)*3000;

[Archit Yadav]

Did you get any solution to this problem?

[Prateek Tiwari]

Even I am looking for the answer to this question.

[Sergey Kasyanov]

The same problem.

[David Goodmanson]

For function inputs Q and T, there are going to be problems when Q and (T-Ti) are of opposite sign. Is there any explicit or implied way to prevent this, prior to the function call?