Single duffing oscillator subjected to a ground motion.

I'm tying to solve a duffing oscillator subjected to a ground motion(EOM, m.u'' + c.u' + k.u +k3.u^3 = -m.ag ). It gives an error:

Error using odearguments (line 95)

@(T,U)ODE_FUN(T,U,K,M,C,K3,A) returns a vector of length 2002, but the length of initial conditions vector is 2. The vector returned by @(T,U)ODE_FUN(T,U,K,M,C,K3,A) and the initial conditions vector must have the same number of elements.

Error in ode45 (line 115)

odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

Error in Cntrl (line 47)

[t,u] = ode45(@(t,u) ode_fun(t,u,k,m,c,k3,a),tspan,[u0; v0]);

k   = 10;
k3 = 100;
m  =0.2533;
c   = 0.1592;
u0=0;
v0=0;
a = Ag_elcentro_FAM; %% EQ ground motion
dt=0.01;
tspan = [0:dt:20]';

ode_fun = @(t,u,k,m,c,k3,ag)[u(2);-(k/m)*u(1)-(c/m)*u(2)-(k3/m)*u(1)^3-ag];

[t,u] = ode45(@(t,u) ode_fun(t,u,k,m,c,k3,a),tspan,[u0; v0]);
disp=u(:,1);
vel=u(:,2);