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

Question

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