Validation issue on custom combobox

Hi everyone,
I am using a custom component which provides autocomplete feature.the component is from flextraautocomplete nd i m using flex 4 version
The problem is while validating the autocomplete box the textinput of combobox always remains red ..evn after successful validation.
Any idea pls .. why such behaviour .. Flextra autocomplete is mx combobox nd mx textinput is programmitaclly added in it.
I tried errorString = ' ' ..but to no avail
Please help me in understanding this behaviour
and provide a possible solution

Thanks

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