diff --git a/src/Spout/Common/Helper/CellTypeHelper.php b/src/Spout/Common/Helper/CellTypeHelper.php
index ab71a25..68ab0db 100644
--- a/src/Spout/Common/Helper/CellTypeHelper.php
+++ b/src/Spout/Common/Helper/CellTypeHelper.php
@@ -73,6 +73,6 @@ class CellTypeHelper
      */
     public static function isFormula($value)
     {
-        return (strpos($value, '=') === 0);
+        return (is_array($value) || strpos($value, '=') === 0);
     }
 }
diff --git a/src/Spout/Writer/XLSX/Manager/WorksheetManager.php b/src/Spout/Writer/XLSX/Manager/WorksheetManager.php
index f60d873..341d46f 100644
--- a/src/Spout/Writer/XLSX/Manager/WorksheetManager.php
+++ b/src/Spout/Writer/XLSX/Manager/WorksheetManager.php
@@ -229,7 +229,11 @@ EOD;
         $cellXML .= ' s="' . $styleId . '"';
 
         if ($cell->isFormula()) {
-            $cellXML .= ' ><f>' . substr($cell->getValue(), 1) . '</f></c>';
+            if (is_array($cell->getValue())) {
+                $cellXML .= ' ><f>' . substr($cell->getValue()[0], 1) . '</f><v>' . $cell->getValue()[1] . '</v></c>';
+            } else {
+                $cellXML .= ' ><f>' . substr($cell->getValue(), 1) . '</f></c>';
+            }
         } elseif ($cell->isString()) {
             $cellXML .= $this->getCellXMLFragmentForNonEmptyString($cell->getValue());
         } elseif ($cell->isBoolean()) {