SSRS Report Parameter Default Null Value not Applied

I have a SSRS report with a single integer parameter.

Using Report Builder I've:

• changed the parameter to allow null values.
• set the default value to Null.
• saved the report.

However, when the report is requested from the server, the parameter is empty (rather than Null) and the Null checkbox isn't ticked.

Even though the report was being saved with the change, this wasn't being observed when it was rendered and returned by the server.

I had to use 'Save As' to save the report as a different name to get the change to apply. I could then delete the original report and rename the revised report to that of the original.