Use object output from extrinsic functions within embedded matlab function in simulink

Hi, I am facing a problem similar to this Stackoverflow Question. I wish to use the MPT3 function "Polyhedron" within embedded matlab function. A minimal working example is below.

     [A,b] = fcn(vertices)
     % vertices is a n-times-2 matrix

     coder.extrinsic('Polyhedron');

     coder.varsize('A',[1024,2]);
     coder.varsize('b',[1024 1]);

     A = zeros(1024,2);
     b = zeros(1024,1);

     P = Polyhedron(vertices);

     A = P.A;
     b = P.b;

However, I get the error

• Attempt to extract field 'A' from 'mxArray'.
• Attempt to extract field 'b' from 'mxArray'.

when I try to compile my simulink model. How should I pre-initialise the object 'P'?

Thank you!...