Frontend status of Action Script type Webhook is always failed

Steps to reproduce:

1. Create Global script with scope Action operation, Type Webhook, with only one line " return 'OK';"
2. Create Action to run that script
3. Create test trigger to start that Action

Result:

Script is executed, but in frontend there is status failed with error: cannot execute script: (null)

Expected:
Status have to be success.
See attached screenshots...

In real scenario I am using real script which works as expected, but frontend result is always failed. Maybe I have to return some other text or result code, but I tried many and never succeeded. I did not find solution anywhere in Zabbix documentation.