FontStruct error when starting emacs

[Ufuk Kirik]

Hi,

I am currently using Debian sid distro with 2.9.1 kernel.. I have this
problem with emacs, which is that when i start emacs, all the characters
in the screen and on the menus of emcas appear as empty rectangles. On
the terminal window i get this error message:

Warning: Cannot convert string
"-*-helvetica-medium-r-*--*-120-*-*-*-*-iso8859-1" to type FontStruct

how can i fix this issue? 

Thanks, and best regards