Error in optimisation code (division by an OptimizationVariable not supported.in prob.objective)

I am currently building a basic optimisation code to maximise d and t within specific constraints. Once the code works I will develop ity further but for now I am stuck with the error Division by an OptimizationVariable not supported. After reviewing this on the forums I am still no closer to working out what I am doing wrong. Any advice would be appreciated. Thank you.

%% Clear workspace
clear;
clc;

%% Input Variables
l=14830; % Member length (mm)
k=1;     % Effective length factor


%% Equations
prob = optimproblem('ObjectiveSense','max');

d = optimvar('d', 1,'LowerBound',0);
t = optimvar('t',1,'LowerBound',0);
prob.Objective= (d/t) + ((k*l)/r);
%% Variables
din=d-(2*t);                   % Member inner diameter
a=3.1415/4 * ((d^2)-(din^2));  % Member cross sectional area
i=3.1415/64 * ((d^4)-(din^4)); % Member second moment of inertia
r=(a/i)^0.5;                   % Member radius of gyration

%% Constraints
cons1 = d>=0;
cons2 = d<=3000;
cons3 = t>=0;
cons4 = t<=100;
cons5 = d/t<=30;
cons6 = k*l/r<=40;

prob.Constraints.cons1 = cons1;
prob.Constraints.cons2 = cons2;
prob.Constraints.cons3 = cons3;
prob.Constraints.cons4 = cons4;
prob.Constraints.cons5 = cons5;
prob.Constraints.cons6 = cons6;

show(prob)

sol = solve(prob);

sol.d
sol.t

The error I am getting is:

Error using optim.internal.problemdef.Rdivide.getRdivideOperator

Division by an OptimizationVariable not supported.

Error in /

Error in optimizejacketframe (line 15)

prob.Objective= (d/t) + ((k*l)/r);

[Jeffrey Clark]

@Maximilian, both d and t have optimvar LowerBound set to 0 (same as cons1 and cons3), cons5 = d/t<=30 and prob.Objective= (d/t) + ((k*l)/r) seem to set up a condition to divide by zero t. Perhaps not limiting t to be greater than zero can be the cause of the error and would likely be an issue anyway?

[Walter Roberson]

r is not defined.

In current MATLAB versions, if you define r with a definite numeric value, the code is accepted.

I notice the poster is using R2018b; it might be necessary to upgrade in order to use division by an optimization variable (even one constrained to be non-zero)

[Maximilian]

Amazing thank you for both of your help. Now that I have updated my matlab version the code runs up to the sol = solve(prob) section. Now it is saying Error using optim.problemdef.OptimizationProblem/solve SOLVE requires a non-empty initial point structure to solve a nonlinear problem. So I now assume I need to set an initial guess for the code to run so I will get on with that.