Fast cast starting off with straight line

Hello, so I’m trying to calculate acceleration with fastcast. Everything almost works fine, but at the beginning it’s a straight line and then starts accelerating. Does anyone know how to fix this? Look below for image:

My script:

local fastcast = require(game.ReplicatedStorage.FastCastRedux)
local thing =

fastcast.VisualizeCasts = true
local behavior = fastcast.newBehavior()
behavior.Acceleration =, 10, 10)
thing:Fire(script.Parent.WorldPosition, nil,script.Parent.WorldCFrame.LookVector * (behavior.Acceleration / 2) , behavior)

Any help is appreciated. Thanks in advance