GDB error Value can't be converted to integer
===

I am trying to debug some application and i want to pass some data using following command but i am getting error.
   
        ....
        ....
        (gdb) x/20x $esp
        Value can't be converted to integer.
        (gdb)

My GDB version is followin: 

    #gdb -v
    GNU gdb (GDB) Red Hat Enterprise Linux (7.0.1-23.el5)
    Copyright (C) 2009 Free Software Foundation, Inc.
    License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
    This is free software: you are free to change and redistribute it.
    There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
    and "show warranty" for details.
    This GDB was configured as "x86_64-redhat-linux-gnu".
    For bug reporting instructions, please see:
    <http://www.gnu.org/software/gdb/bugs/>.

