How do I launch a process with low priority? C#

I want to execute a command line tool to process data. It does not need to be blocking. I want it to be low priority. So I wrote the below

 Process app = new Process();
 app.StartInfo.FileName = @"bin\convert.exe";
 app.StartInfo.Arguments = TheArgs;
 app.PriorityClass = ProcessPriorityClass.BelowNormal;
 app.Start();

However, I get a System.InvalidOperationException with the message "No process is associated with this object." Why? How do I properly launch this app in low priority?

Without the line app.PriorityClass = ProcessPriorityClass.BelowNormal; the app runs fine.