object:prepare("anotherSet") adds a blank frame? huh?

I have searched the forum, but the result was hard to understand. If anyone can explain to me why this happens and what I can do to solve it, it’d be nice.

I have a sprite sheet of 40 images. 20 of them walks to the right and the other 20 walks to the left.

I have them loaded:

sprite.add (walkingSprite, “normal_Right”, 1,20, 500, 0)

sprite.add (walkingSprite, “normal_Left”, 21,40, 500, 0)

It works perfectly when I load it in initially to be played.

object:prepare ("normal_Right)

object:play

but when I load in the second set of order to walk left…it keeps adding a blank frame to the animation

object:prepare ("normal_Left)

object:play

Why is that?