diff --git a/classes/report_formats/XlsReportBase.php b/classes/report_formats/XlsReportBase.php
index ee07c3b3..720e40b2 100644
--- a/classes/report_formats/XlsReportBase.php
+++ b/classes/report_formats/XlsReportBase.php
@@ -3,7 +3,8 @@ abstract class XlsReportBase extends ReportFormatBase {
 	private static function columnLetter($c){
 		$c = intval($c);
 		if ($c <= 0) return '';
-
+		$letter = '';
+		
 		while($c != 0){
 			$p = ($c - 1) % 26;
 			$c = intval(($c - $p) / 26);