Glitch with velocity

[Duck]

Code:

gamePlayer = script.Parent.Parent.Parent

function firebullet()
Bullet = game.Environment.Bullet
link(script.Parent.MouseDown, firebullet)
gunPosition = script.Parent:getPosition()
gunAngle = script.Parent:getAngle()
newBullet = Bullet:clone()
newBullet.Parent = game.World
newBullet.Position = gunPosition
newBullet.Rotation = gunAngle - 180
newBullet.FixedVelocity.Velocity = -100 --error here
end

link(script.Parent.MouseDown, firebullet)