diff --git a/apps/server/src/api-integration/__test__/integration.utils.test.ts b/apps/server/src/api-integration/__test__/integration.utils.test.ts
new file mode 100644
index 000000000..4b9c6a1d6
--- /dev/null
+++ b/apps/server/src/api-integration/__test__/integration.utils.test.ts
@@ -0,0 +1,91 @@
+import { EntryCustomFields, OntimeEvent } from 'ontime-types';
+import { isValidChangeProperty } from '../integration.utils.js';
+
+describe('isValidChangeProperty()', () => {
+  test('correct value and property', () => {
+    const testEvent = {
+      id: 'test',
+      duration: 111,
+    } as OntimeEvent;
+
+    expect(isValidChangeProperty(testEvent, 'duration', 123)).toBeTruthy();
+  });
+
+  test('correct property and undefined value', () => {
+    const testEvent = {
+      id: 'test',
+      duration: 111,
+    } as OntimeEvent;
+
+    expect(isValidChangeProperty(testEvent, 'duration', undefined)).toBeFalsy();
+  });
+
+  test('missing property and undefined value', () => {
+    const testEvent = {
+      id: 'test',
+      duration: 111,
+    } as OntimeEvent;
+
+    expect(isValidChangeProperty(testEvent, 'missing', 123)).toBeFalsy();
+  });
+
+  test('non existing custom value', () => {
+    const testEvent = {
+      id: 'test',
+      duration: 111,
+      custom: {
+        field: 'test',
+      } as EntryCustomFields,
+    } as OntimeEvent;
+
+    expect(isValidChangeProperty(testEvent, 'custom:test', 123)).toBeFalsy();
+  });
+
+  test('existing custom value', () => {
+    const testEvent = {
+      id: 'test',
+      duration: 111,
+      custom: {
+        test: 'test',
+      } as EntryCustomFields,
+    } as OntimeEvent;
+
+    expect(isValidChangeProperty(testEvent, 'custom:test', 123)).toBeTruthy();
+  });
+
+  test('empty custom definition', () => {
+    const testEvent = {
+      id: 'test',
+      duration: 111,
+      custom: {
+        test: 'test',
+      } as EntryCustomFields,
+    } as OntimeEvent;
+
+    expect(isValidChangeProperty(testEvent, 'custom:', 123)).toBeFalsy();
+  });
+
+  test('build-in in custom', () => {
+    const testEvent = {
+      id: 'test',
+      duration: 111,
+      custom: {
+        test: 'test',
+      } as EntryCustomFields,
+    } as OntimeEvent;
+
+    expect(isValidChangeProperty(testEvent, 'custom:toString', 123)).toBeFalsy();
+  });
+
+  test('build-in in top object', () => {
+    const testEvent = {
+      id: 'test',
+      duration: 111,
+      custom: {
+        test: 'test',
+      } as EntryCustomFields,
+    } as OntimeEvent;
+
+    expect(isValidChangeProperty(testEvent, 'toString', 123)).toBeFalsy();
+  });
+});
diff --git a/apps/server/src/api-integration/integration.controller.ts b/apps/server/src/api-integration/integration.controller.ts
index 250cd15b0..52290fade 100644
--- a/apps/server/src/api-integration/integration.controller.ts
+++ b/apps/server/src/api-integration/integration.controller.ts
@@ -18,7 +18,7 @@ import { validateMessage, validateTimerMessage } from '../services/message-servi
 import { runtimeService } from '../services/runtime-service/runtime.service.js';
 import { eventStore } from '../stores/EventStore.js';
 import * as assert from '../utils/assert.js';
-import { parseProperty } from './integration.utils.js';
+import { parseProperty, isValidChangeProperty } from './integration.utils.js';
 import { socket } from '../adapters/WebsocketAdapter.js';
 import { throttle } from '../utils/throttle.js';
 import { coerceEnum } from '../utils/coerceType.js';
@@ -77,10 +77,9 @@ const actionHandlers: Record<ApiActionTag, ActionHandler> = {
     let shouldThrottle = false;
 
     Object.entries(data).forEach(([property, value]) => {
-      if (typeof property !== 'string' || value === undefined || !(property in targetEntry)) {
+      if (!isValidChangeProperty(targetEntry, property, value)) {
         throw new Error('Invalid property or value');
       }
-      // parseProperty is async because of the data lock
       const newObjectProperty = parseProperty(property, value);
       const key = Object.keys(newObjectProperty)[0];
       shouldThrottle = shouldThrottle || willCauseRegeneration(key);
diff --git a/apps/server/src/api-integration/integration.utils.ts b/apps/server/src/api-integration/integration.utils.ts
index 53a377441..d2bb401c3 100644
--- a/apps/server/src/api-integration/integration.utils.ts
+++ b/apps/server/src/api-integration/integration.utils.ts
@@ -1,4 +1,4 @@
-import { EndAction, TimeStrategy, TimerType, isKeyOfType } from 'ontime-types';
+import { EndAction, OntimeEntry, TimeStrategy, TimerType, isKeyOfType } from 'ontime-types';
 import { maxDuration } from 'ontime-utils';
 
 import { coerceBoolean, coerceColour, coerceEnum, coerceNumber, coerceString } from '../utils/coerceType.js';
@@ -60,3 +60,14 @@ export function parseProperty(property: string, value: unknown) {
   const parserFn = propertyConversion[property];
   return { [property]: parserFn(value) };
 }
+
+export function isValidChangeProperty(target: OntimeEntry, property: string, value: unknown): boolean {
+  if (typeof property !== 'string') return false;
+  if (value === undefined) return false;
+  if (property.startsWith('custom:') && 'custom' in target) {
+    const customProperty = property.slice('custom:'.length);
+    if (!customProperty) return false;
+    return Object.hasOwn(target.custom, customProperty);
+  }
+  return Object.hasOwn(target, property);
+}