How to fix an invalidated key frame sequence in Creo Parametric animation?

I created several snapshots to make an animation with in Creo Parametric.  I created the key frame sequence, but when I try to play the animation I keep getting the following error "Key frame sequence Kfs1 invalidated and will be ignored."  What does this mean?  How did I get an invalidated key frame sequence and how can I correct it?  Has anyone had this problem before?  I am not sure where to start because I did the process exactly as I have before, and it worked previously.