'Display as' box in Contacts

In my Contacts, when inputting a name (eg Derek) in the ‘Display as’ box, I get error message :  value “Derek” <full email address is displayed here>  is not a valid email address. Please review the address and the associated display name. I know for certain that the email address is correct and valid so what seems to be the problem? This only happens on random contacts. Thanks for your help.