how to use substr in a conditional formatting

Trying to highlight a row in a crosstab section if the first element in a row starts with 'HONDA'. Created crosstab using RTF template wizard. Then added conditional formatting and modified it as follows:<?if@row:substr(./H,1,5)='HONDA'?><?attribute@incontext:background-color;'#80FFFF'?><?end if?>.

Getting an error when trying to preview. Report renders fine if I remove conditional formatting.