**Problem :**

I am facing an error for the BC in this equation:

`a = NDSolve[{D[h[t, x], t] + Sin[x Degree] h[t, x]^2 D[h[t, x], x] + 2/3 Cos[x Degree] h[t, x]^3 == 0, h[0, x] == 1, D[h[t, 0], x] == 0}, h, {t, 0, 100}, {x, 0, 90}]`

`NDSolve::deqn: Equation or list of equations expected instead of True in the first argument {2/3 Cos[[Degree] x] h[t,x]^3+h[t,x]^2 Sin[[Degree] x] (h^(0,1))[t,x]+(h^(1,0))[t,x]==0,h[0,x]==1,True}. >>`

Any tips to resolve it?