BaseListError

[Tanmay]

I have a checkbox, onvalue event of it i select diffrent items from combobox. This is giving an error:

ERROR @base/baselist.lzx#366: call to undefined method 'getItemByIndex'

<combobox width="80" id="cmbDoc">
<textlistitem>abc</textlistitem>
<textlistitem>bcd</textlistitem>
<textlistitem>efg</textlistitem>
<textlistitem>hij</textlistitem>
</combobox>
<checkbox id="chk" text="abc">
<handler name="onvalue">
if(this.value)
{
cmbDoc.selectItemAt(1);
}
else
{
cmbDoc.selectItemAt(3);
}
</handler>
</checkbox>

Suprisingly if the else part of the code is commented the error does not come.
Any Ideas why this error is coming?