String ('char') variables from symbolic expressions

I'm using this code for numerical Laplace inversions: http://www.mathworks.com/matlabcentral/fileexchange/32824-numerical-inversion-of-laplace-transforms-in-matlab

The code takes in symbolic expressions, i.e., 'char' variables (e.g. F = 'sin(s)'). I need to build my F from numerical data stored in 'double' type variables. Why does the following not work? Any help appreciated.

if true
  function [t1 ft1] = laplaceinseries(sn,g1,greal1,g2,greal2)
%sn,g1,greal1,g2,greal2 are numerical vectors ('double' type variables)
syms s F1 F2 F3
% F1(s), F2(s) and F3(s) in symbolic form
for j = 1:length(sn)
  F1 = F1 + (greal1(j) - greal2(j))/(s+1/sn(j));
  F2 = F2 + (greal1(j) + greal2(j))/(s+1/sn(j));
  F3 = F3 + (g1(j)- g2(j))/(s+1/sn(j));
end
F = F1/F2*F3;
% F(s) as a string
F=char(F);                     
[t1,ft1]=INVLAP(F,0.1,500,1000,6,39,89);
end

[Walter Roberson]

Where did you initialize F1, F2, F3 ?

Answer 1

[Tom]

Thank you so much, Walter! I did not know that this was necessary, but a proper symolic initialization solved the problem perfectly: F1 = sym(0); F2 = sym(0); F3 = sym(0);