[ubuntu] apt-file wants CDROM

I tried to install apt-file via "apt-get install apt-file" and got the message:
Put CDROM labeled [Ubuntu_12.04.2_LTS__Precise_Pangolin__-_Release_i386_(20130213)] in the cdrom device and press [ENTER]

As I have got neither CDROM nor CD-drive (installed from USB-stick) I installed apt-file some other way.
However, when I try "apt-file update", I get again the request to insert the CD.

How to get around this?

Regards,

ulrich