nlapiGetLineItemText giving null value on Save Record function.

nlapiGetLineItemText is giving null value when written on Save Record function.But the same code is giving the correct values when written on Field change function.
Any ideas?

for (var i=1; i<=count; i++)      {          var unitVal = nlapiGetLineItemText('item','units',i);          nlapiSetLineItemValue('item','custcol_units_text',i,unitVal);       }

Thanks,
Iby