Reset default intensity to zero

[Simonkbike]

I was changing defaults on my moving head lights and now fixture #5 has a Default intensity value of 5.  (ie. on the source screen it has yellow "D") I tried re-saving it to zero but it won't stick, reverting back to 5 when I clear, clear.

Settings are as shown, and I've tried variations of tagged vs selected fixtures. 

Any suggestions what I'm doing wrong? I don't think any other option for the yellow D applies.

Simon.