Gif Not Saving Correctly

I've been making gifs for a couple months, and after I finish I export them via Save For Web (Legacy). There I hit save, name the file, and I'm done. However, I did this again and the gif saved, but when I try to open it, it says "The file “Morph.gif” could not be opened." it gives the reason "It may be damaged or use a file format that Preview doesn’t recognize." However, I can still open other gifs and it is a gif file.