Error using ==> surf at 78 Z must be a matrix,not a scalar o

Error using ==> surf at 78 Z must be a matrix,not a scalar or vector.怎么将i变为矩阵那
delta=0:0.1:1;
epsilon=0:0.1:1;
[xx,tt] = meshgrid(epsilon,delta);
i=-2*((1-delta).*(1-epsilon).*log2(1/2*(1-delta).*(1-epsilon))+delta.*(1-epsilon).*log2(1/2*delta.*(1-epsilon))+(1-delta).*epsilon.*log2(1/2*(1-delta).*epsilon)+delta.*epsilon.*log2(1/2*delta.*epsilon));
surf(xx,tt,i);