MATLAB: Unknown parameters in ode

ode

I am trying to solve an ode with an unknown parameter. I am not sure If I am doing it in the right way. 'Not enough input arguments'
tol = 10^-3; % Treshold
MAX = 100;
H = 0; 
interval = [0.01 0.2]; % alpha interval 
a = interval(1);
b = interval(2);
alpha = (a+b)/2;
v0=[1 0.01 1];
[z,v]=ode45(@rhs,interval,v0,alpha);
iter = 1;
while(abs((v(end, 2)) - H) > tol)
alpha= (a + b)/2;
[z,v]=ode45(@rhs,zspan,v0,alpha);
end
if(v(end, 2) - H > 0)
b = alpha;
else
a = alpha;
end
iter = iter + 1;
if(iter > MAX)
return;
end
function parameters=rhs(z,v,alpha)
          da= 2*alpha-(v(1).*v(3))./(2*v(2).^2);
          db= (v(3)./v(2))-(2*alpha*v(2)./v(1));
          dc= -(2*alpha*v(3)./v(1)); 
 parameters=[da;db;dc];
end
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