Help with metamethod returning nil?

Hi! I've been trying to learn metatables so I thought I'd start off with something basic. The __add works fine, but for some reason, __sub doesn't.

local vec2_meta = {
	__add = function(a,b)
		return Vec2(
			a.x+b.x,
			a.y+b.y
		)
	end,
	__sub = function(a,b)
		return Vec2(
			a.x-b.x,
			a.y-b.y
		)
	end,
	
}

function Vec2(x,y)
	local v = {
		x = x or 0,
		y = y or 0
	}
	
	setmetatable(v,vec2_meta)
	
	return v
end

local a = Vec2(40,29)
local b = Vec2(10,4)
local c = Vec2(0,0)
local d = Vec2(0,0)

c = a + b
d = a - b

print(c.x, c.y) --> prints 50,33
print(d.X, d.Y) --> prints nil,nil

Could you tell me why this doesn't work and how to fix it?