Upgrading SSRS 2014 to 2016 - Index was out of range. Must be non-negative and less than the size of the collection.

I've just tried to upgrade our main SSRS 2014 instance to 2016.  

It failed towards the end of the install with the following error:

The following error has occurred:

Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

For help, click: http://go.microsoft.com/fwlink?LinkID=20476&ProdName=Microsoft%20SQL%20Server&EvtSrc=setup.rll&EvtID=50000&ProdVer=13.0.1601.5&EvtType=0x35C10BED%25400xF538E98B

It looked like SSRS was pretty much installed - but the service would start and stop again.

I've rolled the VM's and database back for the time being.

Anyone come across this when upgrading SSRS before? It looks like this error usually pops up at the report level.  I might try deleting all the reports and trying the upgrade again to rule them out