Intellisense not working in VB Intellisense ExpressionEditor

[pappasa]

The VB expression editor with intellisense does not seem to work.
As shown in image https://ibb.co/6rb8cjH, I create a new Custom (VB Intellisense ExpressionEditor) workflow and add Argument of type String. I add an Assign activity for a = a.SubString(0, 2), but no completions are displayed for a.SubString.