Requirement module with user defined field to show the release module objects

Hi,

I managed to create a user field in my template that will show the releases tree structure (for release objects only) 

but it show an error "Cannot convert the given value to the given type. The type <String> is not supported" when i click to refresh and save, the message appear several times but eventually it save the changes.

any ideas how to get rid of this anoying messege ?

Thanks in advance :) 

Ofir