Impossible to add opaque action in activity diagram

The activity diagram creation have some weird behaviour when used with partitions : it was impossible for me to add any "opaque action" in the partitions. After a while and some researches into the documentation it seems that there is no particular things to do for that. The only workaround I've found is to put an opaque action outside the partition which is allowed and then move it to the partition as shown in the picture.

I'm sure I do something wrong here but what?

You should be able to select an opaque action from the palette and then click in a partition, the opaque action appears directly in the partition. Which version of UML Designer do you use? It seems to work correctly for me with the 7.1.