diff --git a/Classes/PHPExcel/Calculation.php b/Classes/PHPExcel/Calculation.php
index 6159be9..c4f2112 100644
--- a/Classes/PHPExcel/Calculation.php
+++ b/Classes/PHPExcel/Calculation.php
@@ -2640,7 +2640,7 @@ class PHPExcel_Calculation {
 	}	//	function _showTypeDetails()
 
 
-	private static function _convertMatrixReferences($formula) {
+	private function _convertMatrixReferences($formula) {
 		static $matrixReplaceFrom = array('{',';','}');
 		static $matrixReplaceTo = array('MKMATRIX(MKMATRIX(','),MKMATRIX(','))');
 
@@ -2729,7 +2729,7 @@ class PHPExcel_Calculation {
 
 	// Convert infix to postfix notation
 	private function _parseFormula($formula, PHPExcel_Cell $pCell = NULL) {
-		if (($formula = self::_convertMatrixReferences(trim($formula))) === FALSE) {
+		if (($formula = $this->_convertMatrixReferences(trim($formula))) === FALSE) {
 			return FALSE;
 		}