Sellect All Checkboxes And Inverse Selection

I have this table with all my values I want to check and inverse:

HTML Code:

<input type=text name="set_name" value="" maxlength=255 size="50"><br><br>
<input type=submit value="Submit"> <input type=reset value="Reset"><br><br>
Select members belonging to this set.<br>
<input type=button value="Check All" onClick="this.value=check(this.form.set_member[])">
<table>
<td><input type=checkbox name="set_member[]" value="77"> Adam Green</td>
<td><input type=checkbox name="set_member[]" value="70"> Brenton Rzepecki</td>
<td><input type=checkbox name="set_member[]" value="94"> Brett Stayt</td>
<td><input type=checkbox name="set_member[]" value="63"> Brock Wallis</td>
</tr>
<tr>
<td><input type=checkbox name="set_member[]" value="102"> Chris Cornish</td>
<td><input type=checkbox name="set_member[]" value="68"> Dale Smulders</td>
<td><input type=checkbox name="set_member[]" value="114"> Damian Astone</td>
<td><input type=checkbox name="set_member[]" value="106"> Dave Chang</td>
</tr>
<tr>
<td><input type=checkbox name="set_member[]" value="101"> David Waldron</td>
<td><input type=checkbox name="set_member[]" value="84"> Doug Thomson</td>
<td><input type=checkbox name="set_member[]" value="108"> Gary Bruecher</td>
<td><input type=checkbox name="set_member[]" value="87"> Gerald Delic</td>
</tr>
<tr>
<td><input type=checkbox name="set_member[]" value="104"> Gordon Hay</td>
<td><input type=checkbox name="set_member[]" value="86"> Jamie Vw</td>
<td><input type=checkbox name="set_member[]" value="99"> Jim Felvus</td>
<td><input type=checkbox name="set_member[]" value="88"> Jordan Masters</td>
</tr>
<tr>
<td><input type=checkbox name="set_member[]" value="71"> Josh P</td>
<td><input type=checkbox name="set_member[]" value="89"> Josh Taylor</td>
<td><input type=checkbox name="set_member[]" value="96"> Karis Putland</td>
<td><input type=checkbox name="set_member[]" value="116"> Kevin Craig</td>
</tr>
<tr>
<td><input type=checkbox name="set_member[]" value="65"> Kyle Putland</td>
<td><input type=checkbox name="set_member[]" value="73"> Logan Bruecher</td>
<td><input type=checkbox name="set_member[]" value="118"> Michael Newson</td>
<td><input type=checkbox name="set_member[]" value="110"> Mike Nyman</td>
</tr>
<tr>
<td><input type=checkbox name="set_member[]" value="85"> Mitch Virlee</td>
<td><input type=checkbox name="set_member[]" value="54"> Nathan Rzepecki</td>
<td><input type=checkbox name="set_member[]" value="75"> Patrick Green-Mackinlay</td>
<td><input type=checkbox name="set_member[]" value="83"> Patryk S</td>
</tr>
<tr>
<td><input type=checkbox name="set_member[]" value="93"> Paul Smith</td>
<td><input type=checkbox name="set_member[]" value="113"> Peter Needham</td>
<td><input type=checkbox name="set_member[]" value="90"> Ricardo Velazquez</td>
<td><input type=checkbox name="set_member[]" value="109"> Rick Penlington</td>
</tr>
<tr>
<td><input type=checkbox name="set_member[]" value="111"> Rik Christiansen</td>
<td><input type=checkbox name="set_member[]" value="69"> Rob Ralston</td>
<td><input type=checkbox name="set_member[]" value="91"> Rohan Martin</td>
<td><input type=checkbox name="set_member[]" value="107"> Samuel Silvestro</td>
</tr>
<tr>
<td><input type=checkbox name="set_member[]" value="97"> Scott Kerenyi</td>
<td><input type=checkbox name="set_member[]" value="56"> Steve Smulders</td>
<td><input type=checkbox name="set_member[]" value="92"> Steve Pugh</td>
<td><input type=checkbox name="set_member[]" value="103"> Stewart Whitson</td>
</tr>
<tr>
<td><input type=checkbox name="set_member[]" value="100"> Stuart Moss</td>
<td><input type=checkbox name="set_member[]" value="112"> Stuart Broom</td>
<td><input type=checkbox name="set_member[]" value="76"> Suhud Simbolon</td>
<td><input type=checkbox name="set_member[]" value="74"> Tim Muhl</td>
</tr>
<tr>
<td><input type=checkbox name="set_member[]" value="115"> Toby Reeve</td>
<td><input type=checkbox name="set_member[]" value="98"> William Reynolds</td>

this is the javascript im using

<SCRIPT LANGUAGE="JavaScript">
<!-- Begin
var checkflag = "false";
function check(field) {
if (checkflag == "false") {
for (i = 0; i < field.length; i++) {
field[i].checked = true;}
checkflag = "true";
return "Uncheck All"; }
else {
for (i = 0; i < field.length; i++) {
field[i].checked = false; }
checkflag = "false";
return "Check All"; }
}
// End -->
</script>
But I keep getting this error

Event thread: onclick
Error:
name: TypeError
message: Statement on line 6: Expression evaluated to null or undefined and is not convertible to Object: field
Backtrace:
Line 6 of inline#1 script in http://radiorally.homelinux.net/goto/add_mass_email_set
for (i = 0; i < field.length; i++)
In unknown script
this.value = check(this.form.set_member);
At unknown location
{event handler trampoline}


Event thread: onclick
Error:
name: TypeError
message: Statement on line 6: Expression evaluated to null or undefined and is not convertible to Object: field
Backtrace:
Line 6 of inline#1 script in http://radiorally.homelinux.net/goto/add_mass_email_set
for (i = 0; i < field.length; i++)
In unknown script
this.value = check(this.form.set_member);
At unknown location
{event handler trampoline}

Can anyone help me sort this out? Why is it throwing this error?