diff --git a/src/Spout/Reader/XLSX/Manager/StyleManager.php b/src/Spout/Reader/XLSX/Manager/StyleManager.php
index d5db0d5..38aa7c8 100644
--- a/src/Spout/Reader/XLSX/Manager/StyleManager.php
+++ b/src/Spout/Reader/XLSX/Manager/StyleManager.php
@@ -175,7 +175,7 @@ class StyleManager
                 $normalizedNumFmtId = ($numFmtId !== null) ? (int) $numFmtId : null;
 
                 $applyNumberFormat = $xmlReader->getAttribute(self::XML_ATTRIBUTE_APPLY_NUMBER_FORMAT);
-                $normalizedApplyNumberFormat = ($applyNumberFormat !== null) ? (bool) $applyNumberFormat : null;
+                $normalizedApplyNumberFormat = ($applyNumberFormat !== null) ? (bool) $applyNumberFormat : false;
 
                 $this->stylesAttributes[] = [
                     self::XML_ATTRIBUTE_NUM_FMT_ID          => $normalizedNumFmtId,