Dynamically specify script to be called by External Tool Node


I want to call a Python script through the External Tool node from the Lab Extensions. I want the actual script to be called to be specified by a String Input Quickform node. However when I set the FlowVariable "command_text" or "pathToExecutable" in the External Tool node configuration and leave the commandLine textarea on the first tab blank, I receive an error saying that it should not be blank. It seems that the flowVariables are completely ignored. Is that a bug, intended or am I doing something wrong?

In the dialog you have to enter some (valid) value. This get overriden by the flow variable during execution.

Thank you, that works!