Execution Error - System.ArgumentNullException

I am receiving the following error in the very beginning of executing my bot. I have not been able to determine which activity is causing this error, and, therefore, I have been unable to fix it. Any help is greatly appreciated. Thanks!


Source: T-Letter Automation

Message: Value cannot be null.
Parameter name: source

Exception Type: System.ArgumentNullException

An ExceptionDetail, likely created by IncludeExceptionDetailInFaults=true, whose value is:
System.ArgumentNullException: Value cannot be null.
Parameter name: source
at System.Linq.Enumerable.Count[TSource](IEnumerable1 source, Func2 predicate)
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)

@amanda.b.derdiger, which activity is this

You are getting error from there only. The exception that is thrown when a Nothing reference is passed to a method that does not accept it as a valid argument. So here you are passing some argument value but actually it does not have any value in that.