edit

src/PHPExcel/Cell/DefaultValueBinder.php

@@ -87,7 +87,7 @@ class PHPExcel_Cell_DefaultValueBinder implements PHPExcel_Cell_IValueBinder
             return PHPExcel_Cell_DataType::TYPE_STRING;
         } elseif ($pValue instanceof PHPExcel_RichText) {
             return PHPExcel_Cell_DataType::TYPE_INLINE;
-        } elseif ($pValue{0} === '=' && strlen($pValue) > 1) {
+        } elseif (is_string($pValue) && $pValue{0} === '=' && strlen($pValue) > 1) {
             return PHPExcel_Cell_DataType::TYPE_FORMULA;
         } elseif (is_bool($pValue)) {
             return PHPExcel_Cell_DataType::TYPE_BOOL;