diff --git a/PHPExcel/PHPExcel/Calculation.php b/PHPExcel/PHPExcel/Calculation.php
index 5c9fa4d..88bcf48 100755
--- a/PHPExcel/PHPExcel/Calculation.php
+++ b/PHPExcel/PHPExcel/Calculation.php
@@ -2760,7 +2760,7 @@ class PHPExcel_Calculation {
 //echo 'Assessing Expression '.substr($formula, $index),PHP_EOL;
 			$opCharacter = $formula[$index];	//	Get the first character of the value at the current index position
 //echo 'Initial character of expression block is '.$opCharacter,PHP_EOL;
-			if ((isset(self::$_comparisonOperators[$opCharacter])) && (strlen($formula) > $index) && (isset(self::$_comparisonOperators[$formula{$index+1}]))) {
+			if ((isset(self::$_comparisonOperators[$opCharacter])) && (strlen($formula) > $index) && (isset(self::$_comparisonOperators[$formula[$index+1]]))) {
 				$opCharacter .= $formula{++$index};
 //echo 'Initial character of expression block is comparison operator '.$opCharacter.PHP_EOL;
 			}