diff --git a/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php b/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
index 4a608ba..5a50476 100644
--- a/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
+++ b/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
@@ -77,6 +77,7 @@ EOD;
             $format = $style->getNumberFormat();
             $content .= '<numFmt numFmtId="'.$format->getId().'" formatCode="'.$format->getFormatCode().'"/>';
         }
+
         $content .= '</numFmts>';
         return $content;
     }
diff --git a/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php b/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
index 66a13d6..4bf1d82 100644
--- a/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
+++ b/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
@@ -121,8 +121,10 @@ class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
                 $registeredFormatId = $this->styleIdToNumberFormatMappingTable[$registeredStyleId];
                 $this->styleIdToNumberFormatMappingTable[$styleId] = $registeredFormatId;
             } else {
+                $formatId = count($this->registeredNumberFormats);
                 $this->registeredNumberFormats[$serializedFormat] = $styleId;
-                $this->styleIdToNumberFormatMappingTable[$styleId] = count($this->registeredNumberFormats);
+                $this->styleIdToNumberFormatMappingTable[$styleId] = $formatId;
+                $format->setId($formatId);
             }
         } else {
             // The fillId maps a style to a fill declaration