'How to avoid Imagemagick's convert overlay of frames in a gif

I have a bunch of files labelled 1.png and so on. I'm using the following command line to produce a gif animation:

convert -delay 20 *.png animation.gif

But the frames get superposed sequentially. I did not expect this behavior, what might be wrong?

Thanks

Solution 1:^[1]

Option -dispose previous:

convert -dispose previous -delay 20 *.png animation.gif

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Solution	Source
Solution 1	StÃ©phane Laurent