Level 3 assignment 2 error - Invoke Process workflow: Value cannot be null, Parameter name : Expression

Hi Devs.

Struggling with this one here.
I wasn’t even changing anything in the process.xaml file and had a successful run.
After it it stops upon reaching process.xaml invoke method.
It says :

Arguments are passed and are not null.

There isn’t even a variable or argument called “expression”.
I tried deleting and adding Invoke workflow again but it hasn’t helped.

Any ideas anyone?

What is TransactionItem in the calling workflow? Is it a String or a Queue Item or something else?

TransactionItem is a string type variable.


Please try adding breakpoints and run the process in debug mode and post a screenshot of where it breaks please.

It breaks when it reaches the Invoke processs.xaml,
That message was captured during debugging.


  1. Please check if the Invoked process Name , path etc are correct.
  2. The mapping between the arguments , the direction of the arguments, the type of the arguments.

If nothing works try doing the Invoke Workflow File part again.

Done all the above already, mentioned in the 1st post.
As I said the error makes no sense to me.

@Aryx I’m currently hitting the same issue. Would you happen to already have a resolution for this? Thanks!