Failed to delete the LUN

Hi guys

I want to delete a lun that I do not need it, but the following message is showed me:

     failed to delete the following LUN(s):

     LUN1- contained within a storage group

this error occures when a LUN is assigned to a storage goup, but there is not any storage group in my storage.

How can I fix it? do you have the same experience?

device: VNX2 5200

Thank you in advance.