Initilize a Symbolic Toolbox array

I am getting an error in line:

Gup=sym(zeros(2,2))

I am trying to initilize a Symbolic Toolbox array. This equation has worked for me in the past.

Here is the error code:

??? Subscript indices must either be real positive integers or logicals.

Error in ==> CoAndContraMetricTensors at 5
Gup=sym(zeros(2,2))

Here is my m-file program:

syms a_1 a_2 a_3 aup1 aup2 aup3 g_11 g_12 g_13 g_21 g_22 g_23 g_31 g_32 g_33 
A_k=[a_1; a_2; a_3]
Aupk=[aup1; aup2; aup3]
g_=[g_11 g_12 g_13; g_21 g_22 g_23; g_31 g_32 g_33]
Gup=sym(zeros(2,2))
G=det(g_)
for i=1:3
    p=i+1
    if p>3 then
        p=p-3
    end
     r=p+1
    if r>3 then
        r=r-3
    end
     for k=1:3
        s=k+1
        if s>3 then
            s=s-3
        end
         t=s+1
        if t>3 then
            t=t-3
        end
        Gup(i,k)=[g_(p,s) g_(p,t);g_(r,s) g_(r,t)]
     end
end

[Walter Roberson]

duplicates http://www.mathworks.co.uk/matlabcentral/answers/80653-substituting-a-syms-variable-with-another-syms-variable-equation