undefined function variable 'x'

Hi all,
My code doesn't work, I'm pretty sure the logic is correct. it always gives me ' undefined function variable x'. I don't know how to make it work. Seems like the handling function @x.... 
I show the code below. Thanks all!

      syms x

     for i = 2 : 1
         b = 0;
         a = i + 1;
         for j = 1 : i
             b = SP(j,i) * p * PR(j,i) * ((exp(-(R(j,i) + x * dt + d) * dt) + exp(-(R(j,i) + x * dt - d) * dt))) + b;
         end
         H(i) = fsolve(@(x)P(a) - FV * b, 0.1);
         for m = 1 : i
             R(m,a) = R(m,i) + H(i) * dt + d;
             PR(m,a) = exp(-R(m,a) * dt);
         end
         for aa = 2 : 10
             a = aa + 1;
             SP(1,a) = p * SP(1,aa) * PR(1,aa);
             L = SP(1,a);
             for bb = 2 : aa
                 SP(bb,a) =p * SP(bb,bb) * PR(bb,bb) + L;
                 L = SP(bb,a) + L;
             end
             SP(aa+1,a) = p * PR(aa,aa) * SP(aa,aa);
         end
     end

[Geoff Hayes]

Jenny - which line throws the error?