「Value cannot be null. Parameter name: source」error happens when the robot begins

When executing the robot, 「Value cannot be null. Parameter name: source」error occurs. Details below.
This error occurs at the sequence which is the most outside of the workflow.
So as soon as the robot starts, the error occurs and the job stops.

After doing this two operations, the the error began to happen.
・Activities are not put together, so I put them together into some sequences(inside the error-sequence)
・I deleted some variables which I use for debugging and now not necessary at the workflow.

I killed the UiPathStudio and UiPathRobot at the WindowsTaskManager and then restart my PC, only to see the same error.

Do you have any ideas?

The error details;

An ExceptionDetail, likely created by IncludeExceptionDetailInFaults=true, whose value is:
System.ArgumentNullException: Value cannot be null.
Parameter name: source
at System.Data.DataTableExtensions.AsEnumerable(DataTable source)
at lambda_method(Closure , ActivityContext )
at Microsoft.VisualBasic.Activities.VisualBasicValue1.Execute(CodeActivityContext context) at System.Activities.CodeActivity1.InternalExecuteInResolutionContext(CodeActivityContext context)
at System.Activities.Runtime.ActivityExecutor.ExecuteInResolutionContext[T](ActivityInstance parentInstance, Activity1 expressionActivity) at System.Activities.Variable1.PopulateDefault(ActivityExecutor executor, ActivityInstance parentInstance, Location location)
at System.Activities.ActivityInstance.ResolveVariable(Variable variable, ActivityExecutor executor)
at System.Activities.ActivityInstance.ResolveVariables(ActivityExecutor executor)
at System.Activities.Runtime.ActivityExecutor.ExecuteActivityWorkItem.ExecuteBody(ActivityExecutor executor, BookmarkManager bookmarkManager, Location resultLocation)

@ryota Check all the arguments that are being passed to the Workflows. Some may not have the values. Also Try Debugging, It might point you to the right Workflow.

thanks for your kind replying.
does the “arguments” mean variables?
I don’t use any arguments in the error sequense…

or do you mean that all the InvokeWorkflowActivity inside the error sequence need to “import arguments”?

@ryota Yes. If you want to pass values to that workflow which is to be used in that, then you need to Provide arguments to that workflow.

I have this same issue happen intermittently after it starts the “Fetch next transaction” state. But if I restart the bot it works fine. It doesn’t always happen at the start of the day either. Could be running for awhile then crashes.


I made sure the workflow and found out that some InvokeWorkFrowFile-activity needed to pass values which I have already set up, and others didn’t which I haven’t done.

However, I used backup xaml to finalize the workflow, in which the error I faced has never seen. Though the cause of the error doesn’t clear after all, the robot is now made up and works well. So I’m gonna close this thread.

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