Unable to find a post variable called userfile

[eluser]PHP Programmer[/eluser]
I am getting the following error on uploading the files:
"Unable to find a post variable called userfile"

Although, I have assigned the following:

<input type="file" name="userfile1" id="userfile1" />
<td>&lt;input type="file" name="userfile2" id="userfile2" /&gt;&lt;/td>
<td>&lt;input type="file" name="userfile3" id="userfile3" /&gt;&lt;/td>
<td>&lt;input type="file" name="userfile4" id="userfile4" /&gt;&lt;/td>
<td>&lt;input type="file" name="userfile5" id="userfile5" /&gt;
&lt;input type="file" name="userfile6" id="userfile6" /&gt;
&lt;input type="file" name="userfile7" id="userfile7" /&gt;

How can I remove that error??