GroovyCastException while importing Swagger definition

Hi,

I am trying to import a Swagger API definition into an empty project using the Import Swagger feature using SOAPUI 5.3.0 open source version. 

When I do this I get GroovyCastException. I get the same thing whether using a definition created here, or the sample petstore definition http://petstore.swagger.io/v2/swagger.json 

Has anyone else run into this? Has anyone been successful doing this?

Thanks,
Gabor