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Error with [robot_description]

Hi, I am trying to get through a tutorial "Building a Visual Robot Model with URDF from Scratch". I got a problem like this:

Invalid <param> tag: Cannot load command parameter [robot_description]: command [/opt/ros/indigo/share/xacro/ urdf/01-myfirst.urdf] returned with code [1]. 

Param xml is <param command="$(find xacro)/ $(arg model)" name="robot_description"/>
The traceback for the exception was written to the log file

everytime if I want to launch urdf file like this:

roslaunch urdf_tutorial display.launch model:=urdf/01-myfirst.urdf

I have checked the file (at least I think so) and this is what i get:

Expat parsing error.  Check that:
 - Your XML is correctly formed
 - You have the xacro xmlns declaration: xmlns:xacro=""

Traceback (most recent call last):
  File "/opt/ros/indigo/share/xacro/", line 62, in <module>
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/", line 676, in main
    doc = parse(f)
  File "/usr/lib/python2.7/xml/dom/", line 1918, in parse
    return expatbuilder.parse(file)
  File "/usr/lib/python2.7/xml/dom/", line 928, in parse
    result = builder.parseFile(file)
  File "/usr/lib/python2.7/xml/dom/", line 207, in parseFile
    parser.Parse(buffer, 0)
xml.parsers.expat.ExpatError: not well-formed (invalid token): line 1, column 1

Could anyone help me? I would be very grateful.