The REST API to export Cucumber Tests does not work when the filter returns Test Plans

The REST API to export Cucumber Tests does not work when the filter returns Test Plans.

How to reproduce:

1. Create a Test Plan with Cucumbers in it
2. Export the Cucumbers with the REST API /rest/raven/1.0/export/test
3. It returns the error message "Error occurred while exporting raven tests." and does not export any cucumbers.