diff --git a/src/file/numbering/numbering.ts b/src/file/numbering/numbering.ts
index fd2bf0754a..599fc1c8a8 100644
--- a/src/file/numbering/numbering.ts
+++ b/src/file/numbering/numbering.ts
@@ -215,8 +215,8 @@ export class Numbering extends XmlComponent {
         };
 
         const referenceConfigLevels = this.referenceConfigMap.get(reference);
-        const firstLevelStartNumber = referenceConfigLevels?.[0]?.start;
-        if (firstLevelStartNumber) {
+        const firstLevelStartNumber = referenceConfigLevels && referenceConfigLevels[0].start;
+        if (firstLevelStartNumber && Number.isInteger(firstLevelStartNumber)) {
             concreteNumberingSettings.overrideLevel = {
                 num: 0,
                 start: firstLevelStartNumber,