diff --git a/src/Spout/Reader/XLSX/RowIterator.php b/src/Spout/Reader/XLSX/RowIterator.php
index 7716e2c..2bfef30 100644
--- a/src/Spout/Reader/XLSX/RowIterator.php
+++ b/src/Spout/Reader/XLSX/RowIterator.php
@@ -220,7 +220,7 @@ class RowIterator implements IteratorInterface
     {
         // Read dimensions of the sheet
         $dimensionRef = $xmlReader->getAttribute(self::XML_ATTRIBUTE_REF); // returns 'A1:M13' for instance (or 'A1' for empty sheet)
-        if (preg_match('/[A-Z]+\d+:([A-Z]+\d+)/', $dimensionRef, $matches)) {
+        if (preg_match('/^[A-Z]+\d+:([A-Z]+\d+)$/', $dimensionRef, $matches)) {
             $this->numColumns = CellHelper::getColumnIndexFromCellIndex($matches[1]) + 1;
         }