getting Jscript runtime error - Object required

I want to write a function to populate second dropdownlist based on value of first dropdown. I have a function for the event "Onchange". But getting error on below highlighted line like Jscript runtime error-object required when selecting an option from the dropdown. See the code below.
<select name="sys_text" size='1' onchange='populateDD();'> <option value='' selected='selected'>Select System</option> <option value='AFC'>AFC</option> <option value='Telecom'>Telecom</option> </select>
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