Excel Template - Concat function returns error

@Line

Hi,

I'm facing an issue with "concat" function in Excel Template.

If I try to concat static text like below,  it is working fine

<?concat('FIRST_NAME',' ','LAST_NAME')?>

And if I try to concat XML fields like below, excel template is throwing an error

<?concat(FIRST_NAME,' ',LAST_NAME)?>

Error Message:

XML-23006: (Error) value does not match required type

@Line 10 ==> <Cell Index="1" Style="R2C2" Field="XDO_?XDOFIELD5?"><xsl:value-of select="concat(.//FIRST_NAME,' ',.//LAST_NAME)"/>

Any help on this would be highly appreciated.

Thank You!