diff --git a/apps/server/src/api-data/custom-fields/__tests__/customFields.parser.test.ts b/apps/server/src/api-data/custom-fields/__tests__/customFields.parser.test.ts
index de680c234..5d2d004fe 100644
--- a/apps/server/src/api-data/custom-fields/__tests__/customFields.parser.test.ts
+++ b/apps/server/src/api-data/custom-fields/__tests__/customFields.parser.test.ts
@@ -126,4 +126,15 @@ describe('sanitiseCustomFields()', () => {
     const sanitationResult = sanitiseCustomFields(customFields);
     expect(sanitationResult).toStrictEqual(expectedCustomFields);
   });
+
+  it('filters keys that collide with Object prototype properties', () => {
+    const customFields: CustomFields = {
+      toString: { label: 'toString', type: 'text', colour: 'red' },
+      normalField: { label: 'normalField', type: 'text', colour: 'green' },
+    };
+    const sanitationResult = sanitiseCustomFields(customFields);
+    expect(sanitationResult).toStrictEqual({
+      normalField: { label: 'normalField', type: 'text', colour: 'green' },
+    });
+  });
 });
diff --git a/apps/server/src/api-data/custom-fields/customFields.parser.ts b/apps/server/src/api-data/custom-fields/customFields.parser.ts
index 8b0d71002..3d204de8a 100644
--- a/apps/server/src/api-data/custom-fields/customFields.parser.ts
+++ b/apps/server/src/api-data/custom-fields/customFields.parser.ts
@@ -1,5 +1,5 @@
 import { CustomField, CustomFields, DatabaseModel } from 'ontime-types';
-import { checkRegex, customFieldLabelToKey } from 'ontime-utils';
+import { checkRegex, customFieldLabelToKey, isObjectPrototypeKey } from 'ontime-utils';
 
 import type { ErrorEmitter } from '../../utils/parserUtils.js';
 
@@ -45,6 +45,7 @@ export function sanitiseCustomFields(data: object): CustomFields {
       'type' in data &&
       (data.type === 'text' || data.type === 'image') &&
       checkRegex.isAlphanumericWithSpace(data.label) &&
+      !isObjectPrototypeKey(key) &&
       key === customFieldLabelToKey(data.label)
     );
   }
diff --git a/apps/server/src/api-data/custom-fields/customFields.validation.ts b/apps/server/src/api-data/custom-fields/customFields.validation.ts
index 9d374208b..ad27506fd 100644
--- a/apps/server/src/api-data/custom-fields/customFields.validation.ts
+++ b/apps/server/src/api-data/custom-fields/customFields.validation.ts
@@ -1,5 +1,5 @@
 import { body, param } from 'express-validator';
-import { checkRegex } from 'ontime-utils';
+import { checkRegex, customFieldLabelToKey, isObjectPrototypeKey } from 'ontime-utils';
 
 import { requestValidationFunction } from '../validation-utils/validationFunction.js';
 
@@ -9,7 +9,7 @@ export const validateCustomField = [
     .trim()
     .notEmpty()
     .custom((value) => {
-      return checkRegex.isAlphanumericWithSpace(value);
+      return checkRegex.isAlphanumericWithSpace(value) && !isObjectPrototypeKey(customFieldLabelToKey(value));
     }),
   body('type').isIn(['text', 'image']),
   body('colour').isString().trim(),
@@ -24,7 +24,7 @@ export const validateEditCustomField = [
     .trim()
     .notEmpty()
     .custom((value) => {
-      return checkRegex.isAlphanumericWithSpace(value);
+      return checkRegex.isAlphanumericWithSpace(value) && !isObjectPrototypeKey(customFieldLabelToKey(value));
     }),
   body('type').isIn(['text', 'image']),
   body('colour').isString().trim(),
diff --git a/apps/server/src/api-data/rundown/rundown.parser.ts b/apps/server/src/api-data/rundown/rundown.parser.ts
index 332ed29c7..17795f5a9 100644
--- a/apps/server/src/api-data/rundown/rundown.parser.ts
+++ b/apps/server/src/api-data/rundown/rundown.parser.ts
@@ -188,7 +188,7 @@ export function parseRundown(
  */
 export function sanitiseCustomFields(customFields: CustomFields, entry: OntimeEvent | OntimeMilestone | OntimeGroup) {
   for (const field in entry.custom) {
-    if (field in customFields) continue;
+    if (Object.hasOwn(customFields, field)) continue;
     delete entry.custom[field];
   }
   return entry;
diff --git a/apps/server/src/api-data/rundown/rundown.service.ts b/apps/server/src/api-data/rundown/rundown.service.ts
index 48a8873e5..dc2b82907 100644
--- a/apps/server/src/api-data/rundown/rundown.service.ts
+++ b/apps/server/src/api-data/rundown/rundown.service.ts
@@ -16,7 +16,7 @@ import {
   isOntimeEvent,
   isOntimeGroup,
 } from 'ontime-types';
-import { customFieldLabelToKey, getInsertAfterId, resolveInsertParent } from 'ontime-utils';
+import { customFieldLabelToKey, getInsertAfterId, isObjectPrototypeKey, resolveInsertParent } from 'ontime-utils';
 
 import { sendRefetch } from '../../adapters/WebsocketAdapter.js';
 import { getDataProvider } from '../../classes/data-provider/DataProvider.js';
@@ -425,6 +425,9 @@ export async function createCustomField(customField: CustomField): Promise<Custo
   if (!key) {
     throw new Error('Unable to convert label to a valid key');
   }
+  if (isObjectPrototypeKey(key)) {
+    throw new Error('Label conflicts with a reserved field name');
+  }
 
   const { customFields, commit } = createTransaction({ mutableRundown: false, mutableCustomFields: true });
 
@@ -463,7 +466,7 @@ export async function editCustomField(
     mutableCustomFields: true,
   });
 
-  if (!(key in customFields)) {
+  if (!Object.hasOwn(customFields, key)) {
     throw new Error('Could not find label');
   }
 
@@ -472,6 +475,9 @@ export async function editCustomField(
   if (newField.type && existingField.type !== newField.type) {
     throw new Error('Change of field type is not allowed');
   }
+  if (newField.label && isObjectPrototypeKey(customFieldLabelToKey(newField.label))) {
+    throw new Error('Label conflicts with a reserved field name');
+  }
 
   const { oldKey, newKey } = customFieldMutation.edit(customFields, key, existingField, newField);
 
@@ -513,7 +519,7 @@ export async function deleteCustomField(key: CustomFieldKey, projectRundowns: Pr
     mutableRundown: true,
     mutableCustomFields: true,
   });
-  if (!(key in customFields)) {
+  if (!Object.hasOwn(customFields, key)) {
     return customFields;
   }
 
diff --git a/packages/utils/index.ts b/packages/utils/index.ts
index 200f12752..5e4a62c09 100644
--- a/packages/utils/index.ts
+++ b/packages/utils/index.ts
@@ -61,7 +61,7 @@ export { checkRegex, regex } from './src/regex-utils/checkRegex.js';
 export { isColourHex } from './src/regex-utils/isColourHex.js';
 export { splitWhitespace } from './src/regex-utils/splitWhitespace.js';
 
-export { customFieldLabelToKey, customKeyFromLabel } from './src/customField-utils/customFieldUtils.js';
+export { customFieldLabelToKey, customKeyFromLabel, isObjectPrototypeKey } from './src/customField-utils/customFieldUtils.js';
 
 // helpers from externals
 export { deepmerge } from './src/externals/deepmerge.js';
diff --git a/packages/utils/src/customField-utils/customFieldUtils.ts b/packages/utils/src/customField-utils/customFieldUtils.ts
index b8a40821b..cd5c25387 100644
--- a/packages/utils/src/customField-utils/customFieldUtils.ts
+++ b/packages/utils/src/customField-utils/customFieldUtils.ts
@@ -1,5 +1,7 @@
 import type { CustomFields } from 'ontime-types';
 
+const objectPrototypeKeys = new Set(Object.getOwnPropertyNames(Object.prototype));
+
 /**
  * Transforms an alphanumeric label with spaces into a valid key
  */
@@ -7,6 +9,13 @@ export function customFieldLabelToKey(label: string): string {
   return label.trim().replaceAll(' ', '_');
 }
 
+/**
+ * Detects keys that collide with Object prototype properties or methods
+ */
+export function isObjectPrototypeKey(key: string): boolean {
+  return objectPrototypeKeys.has(key);
+}
+
 /**
  * Finds an object key in the CustomFields object that matches the given label
  */