Can't submit iOS app with custom native extension

We've been working on building our first custom ANE - the native extension works great, but when I submit to the iOS app store it gets rejected with the error:-

Invalid Signature - Make sure you have signed your application with a distribution certificate, not an ad hoc certificate or a development certificate. Verify that the code signing settings in Xcode are correct at the target level (which override any values at the project level). If you are certain your code signing settings are correct, choose "Clean All" in Xcode, delete the "build" directory in the Finder, and rebuild your release target.

When I remove the extension, the app does not get rejected so I'm sure it's not the certificate we're using to publish the app itself.  We have tried signing the ANE with a self-signed certificate, or not signing the ANE at all (as apparently signing the ANE is optional).  In either case if we submit the app with the ANE, it gets rejected with the above error.

Can anyone offer any suggestions?