Error when plotting a 3D mesh plot

i keep getting this error "Error using mesh (line 76) Z must be a matrix, not a scalar or vector." everytime i evaluate this

g = 9.8 d = 0:1:10 theta = 0: pi/20 : pi/2 Dc = (d/2).*(1-cos(theta)) a = 2.^(3/2) .* Dc.^(5/2) .* sqrt(g) .*(theta - 0.5.*sin(2.*theta)).^(3/2) b = 8.* sqrt(sin(theta).*(1-cos(theta)).^5/2) Q = a./b x = theta y = d [x,y] = meshgrid(x,y) z = a./b mesh(x,y,z)

i can't find a way to change my Z into a matrix.