plotting – My Plot is empty when I run code

ee = 0.5;
g = 0.1;
gs = Sqrt(g);
f = (Sin(gs*x))^2;
low = gs*Pi/2 ;
up = -low ;
ode = { y''(x) + (2ee - f/g)*y(x) == 0 , y(low)==0 , y(up)==0 } 
sol = NDSolve(ode, y(x), {x, low , up}) 
Plot(y(x) /. sol, {x, low, up})

Boundary conditions are y(low)=y(up)=0. But when I plot this, it shows an empty plot.