Cannot change line item Amount - unexpected error

I have a Server Side Script, running on before submit on Purchase Orders, that is trying to set the Amount of a line item. However, I get an Unexpected Error whenever I try to change the amount to something other than what it already is.

Here's the code:

 function setAllLineItemsPrices() {   var record = nlapiGetNewRecord();   var vendor;   var item;   var qty;   var price;      if(record.getFieldValue('custbody_refresh_all')=='T') {    for(i=1;i&amp;lt;=record.getLineItemCount('item');i++) {     vendor = record.getFieldValue('entity');     item = record.getLineItemValue('item','item',i);     qty = record.getLineItemValue('item','quantity',i);     price = getVolumePrice(vendor,item,qty); //this function fetches a certain unit rate - it works, no problem     if(price!=null) {      record.setLineItemValue( 'item', 'rate', i, price);      <span style="color:'red'">record.setLineItemValue( 'item', 'amount', i, 17250.00);</span>       }    }   }  }