debugging problem with ode45

Error using ==> exist The first input to exist is a string.

??? Error using ==> exist
The first input to exist is a string.

Error in ==> odearguments at 80
if (exist(ode)==2) % M-file

Error in ==> ode45 at 173
[neq, tspan, ntspan, next, t0, tfinal, tdir, y0, f0, odeArgs, odeFcn, ...

Error in ==> co2 at 20
[t,y]=ode45(dy,[0 3000],ones(12,1),options)