you assume moses was assumed just as you assume mary was assumed.
"When Michael the archangel, disputing with the devil, contended about the body of Moses, he durst not bring against him the judgment of railing speech, but said: The Lord command thee." St. Jude 1:9
This quote is from the book The Assumption of Moses, one of the Apocrypha. Jude plainly states that St. Michael contended with the Devil for the body of Moses; obviously it was he, and not the devil, who won:
And after six days Jesus taketh unto him Peter and James, and John his brother, and bringeth them up into a high mountain apart: And he was transfigured before them. And his face did shine as the sun: and his garments became white as snow. And behold there appeared to them Moses and Elias talking with him. (St. Matthew 17:1-3)