Report() takes a hash. It should contain the following keys:

1. Computer: identifies the computer. Can be null for the local computer.
2. Source: Identifies the application.

   This string is not just a text identifier, but also a key in the registry:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\EventLog\Applicat
   +ion\"source".
   [download]

   This is used to associate a compiled resource (a .dll) with the event source (application) that contains a RT_MESSAGETABLE of strings.

   Those strings are identified by an integer ids (EventID), and can contain substitution placeholders (%1, %2 etc).

3. EventType (INFO | WARNING | ERROR | AUDIT SUCCESS |AUDIT FAILURE).
4. Category: An application define numeric value.
5. EventID: The message number within the message table.
6. Data: Some raw binary data associated with the event.
7. Strings: A buffer of 0 or more null-terminated strings that will be substituted for the placeholders embedded within the message text identified by EventID retrieved from the message table located in the resouce dll pointed at in the registry by the source parameter.

The error message is telling you that the "source" identifier cannot be located within the registry. Hence, the text associated with the EventID (which you aren't supplying!) cannot be retrieved. Therefore the string you supply cannot be substituted into it.