Javascript error

[Memnoch1207]

I have a form where I check a ranking of an item, if the items rank is greater than 0, then I want to display a hidden table cell. Here is how I am doing it, but it isn't working.
Can anyone tell me what the problem is???

'The function to show or hide the cell in the table
function CheckRanking(RankValue, elementTag)
{
    if(RankValue >= 1)
    {
        document.forms[0].getElementById(elementTag).style.visibility ='visible';
    }
    else
    {
        document.forms[0].getElementById(elementTag).style.display = 'none';
    }
}
 
'This is the row where the item is ranked
<td class="ESP_Data_Border" ><input maxlength="3" size="4" name="txtPriority" value="0" OnBlur="CheckRanking(this.value, 'ENHANCEMENTVERSION_B4370631-06B8-4DE4-864A-021ACDAD3087');"></td>
 
'This is the cell that should show or hide based on the rank entered into the cell above
<td id="ENHANCEMENTVERSION_B437063106B84DE4864A021ACDAD3087" class="ESP_Data_Border" STYLE="visibility: hidden"><select name="lstVersions"><option>HERE ARE THE VERSIONS</option></select></td>

but I keep getting a "object doesn't support this property or method" error.