GetFeatureSetExtent on polylines

I have configured a workflow using the GetFeatureSetExtent on polygons and it works fine. But when i'm trying to use the same workflow on a polyline the activity doesn't return an extent.

What am i doing wrong since this won't work?

I have tried using Foreach item graphic and Union Task, but I still don't get an extent so there is probably a conversion-problem.

I want to use the extent to make a SetMapExtent.

Rene