Simple Mechanize Issue with tick

Hi,

I have a simple WWW::Mechanize issue that cannot seem to get 
myself.  I am trying to tick a checkbox which is enabled by 
default on the form.  I set the other form values fine, but this 
is the issue checkbox:

<TD class=text height=16><INPUT class=text type=checkbox 
                  CHECKED name=em>Exact Match </TD>

I have tried this in my code:

$mech->tick('em','EXACT MATCH', 'CHECKED');

and just about every other conceivable combination of arguments.

Now, mechanize documentation states:

$mech->tick($name, $value, [$set])
 
'Ticks' the first checkbox that has both the name and value 
assoicated with it on the current form. Dies if there is no 
named check box for that value. Passing in a false value as 
the third optional argument will cause the checkbox to be unticked.

I am sure there is a simple explanation, but it escapes me.

TIA,