DSS won't stack OIII subs?

[JamesF]

I have a large pile of subs, LRGB, Ha and OIII, with darks.  LRGB and Ha stack very happily using a luminance reference frame.  When I come to stack my OIII frames however, they register just fine and then DSS completely fails to stack them without giving any error.  The stacking dialog window pops up and immediately closes again and that's it.

Stretching the OIII frames shows plenty of nice round stars (and I'm sure it would tell me if it wasn't finding enough).

Anyone have any ideas what might be going on?

Thanks,
James

[JamesF]

It seems to be something to do with the reference frame that causes the problem.  If I disable the reference frame then they stack quite happily.  If I pick a few OIII frames at random though, they appear to be a pretty reasonable match for the reference.

James