In Genesis 46:17 (NASB)
And the sons of Asher: Imnah, Ishvah, Ishvi, Beriah, and their sister Serah. (...)
In Numbers 26:44 (NASB)
The sons of Asher by their families: of Imnah, the family of the Imnites; of Ishvi, the family of the Ishvites; of Beriah, the family of the Beriites.
More specifically, in Genesis, Asher has four sons; in Numbers, he only has three clans.
What's the reason for this difference?