Keywordtest with parameter having default value

I have a Keywordtest that has an optinal parameter with a default value (a filename in this case). But when i run my test from the inside the keywordtest editior (run from the selected operation) the parameter seems to be empty. The same behaviour when I run it from the project menue and leave the parameter unassigned. Only if I assign a value and run from the project menue it works as expected. What I'm doing wrong? Or is that a known restriction?

PMM