Deleting a partition in Disk Utility fails. (Error: -69879)

This is the output in Disk Utility: > Disk Utility cannot modify this disk because it contains CoreStorage > physical volumes. Use command line distil instead. > > The selected partition won't be changed. How do I remove the partition? It's completely empty now and I've cleared it by formatting it but I want to completely remove the partition and merge the free space back into the bigger partition. I've tried doing this but it doesn't work in Terminal: diskutil eraseVolume format name device diskutil eraseVolume HFS+ awer disk1s4 It didn't work: Started erase on disk1s4 asdf Unmounting disk Error: -69879: Couldn't open disk