Create lookup column in doc library from a SP list

Hi, 

I have a sharepoint document library containing PDF files. I have added a lookup column to the library which connects to a list on the site (a single column list of single line text type). 

In the document library I can select from the options in the lookup column, but I can not save changes. there is no indication of the error, I simply cant save my changes. 

Can anyone help me with this