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.