Index exceeds array bounds; transfer function model.

I'm modelling the dynamics of an inverted pendulum system. Here's my code:

m=0.1283; M=0.5; I=0.0107; g=9.81; L=0.5;

s=tf('s')

J=m*(L^2)/4+I;
H=M+m;
G=m*g*L/2;
E=m*L/2;
P=J*H-E^2;
format short

sys=1/(P/E*s^2-(E^2+P)*G/(J*E))

Which yields the following error:

Index exceeds array bounds.

Error in sym/subsref (line 859)

R_tilde = builtin('subsref',L_tilde,Idx);

Error in modelo_dominio_s_sin_friccion (line 5)

s=tf('s')

How can I fix it? I'm not sure why there appears to be an error with s=tf('s') when it appears exactly like that in the 'tf' documentation.