Compiling error due to variable size Matrix in simulink (matlab function block)

Looks like multiple people are having trouble with this, none of the workarounds worked for me though.

I am using Matlab 2014b with Simulink 8.4. I am solving a DAE system that describes a turbocharged engine. The system consists of 4 equations, 2 of them are DAEs, 2 are ODEs. For the DAEs I tried using the Algebraic Constraint Block, couldn't get it to simulate correctly though. The two DAEs are of the form: Polynomial of 6th degree in x = 0, and the coefficients are calculated every step of the simulation. I know for a fact that the coefficients are in a range which results in only one real positive root of the polynomial. That is the one I am looking for.

Tried the following code:

function x_4 = fcn(A, B)
c = [1, -1, 2*B, -2*B, B^2, -B^2, -A^2];
r = roots(c);

realR = r(imag(r)==0);
posR = realR(realR>0);
x_4 = posR^0.25;

Error message for this one is

Data 'x_4' is inferred as a variable size matrix, while its specified type is something else.

and

An error occurred while propagating data type 'double' through 'gleichungssystem_poly/Gleichung 4/x_4_calc/A'.

I also tried this code:

function x_4 = fcn(A, B)
c = [1, -1, 2*B, -2*B, B^2, -B^2, -A^2];
r = roots(c);

x_4 = zeros(1);
if isreal(r(1)) && real(r(1))>0
   x_4 = r(1)^0.25;
elseif isreal(r(5)) && real(r(5))>0
    x_4 = r(5)^0.25;
elseif isreal(r(2)) && real(r(2))>0
    x_4 = r(2)^0.25;
elseif isreal(r(3)) && real(r(3))>0
    x_4 = r(3)^0.25;
elseif isreal(r(4)) && real(r(4))>0
    x_4 = r(4)^0.25;
elseif isreal(r(6)) && real(r(6))>0
    x_4 = r(6)^0.25;
end

Tried the whole thing in a for loop as well. Funny thing is it let me compile and simulate the model, however x_4 would never be anything but the initialized 0... I looking into the exact calculations in debugging mode, the condition is fulfilled for one of the roots, nontheless x_4 remains at zero..

I would really appreciate input on this one!