Set random true vpasolve

Hy. I want to solve a system of nonliniar equations and I need multiple solutions. So I try to set random true for the vpasolve function but I get the following error:

Error using getEqnsVars (line 50)
Expecting two arguments: a vector of equations and a vector of variables.

Error in sym/vpasolve (line 95)
[eqns,vars] = getEqnsVars(varargin{1:N});

Error in Untitled (line 10)
[a, b]=vpasolve([diff_f1, diff_f2], [a, b], 'random', true)

The code of my .m file is:

clear
clc
syms a b positive
%equations
f1(a,b)=sin(a+b)*cos(a)/b+a^2*tan(b);
%derivate to get the maximum
diff_f1=diff(f1, 'a');
diff_f2=diff(f1, 'b');
%solve for maximum 
[a, b]=vpasolve([diff_f1, diff_f2], [a, b], 'random', true)

How can I set random true and get more solutions?

[Stephen]

I don't even get the error free answer with the code provided by MATLAB online doc.

I checked my own documentation with R2013a. They didn't offer choice for "random". So I guess "random" is the R2014b thing?