I am trying to use validation by list on a sheet in a workbook.

As the sheet it appears on is designed as a customer interface, and the valid datalist is already on another sheet I would like to be able to set the list source to the other sheet. I have tried to do this several times but the validation selection boxes seems to reject it everytime I try to link to the other sheet.

Is there a way to set this please.
Sheet name Catologue data has the list of data in cells A2:A8
I want to use the list validation on a sheet called Customer Lookup