TC 12.53 debugging parameters

I have a step that works (displays the popup alert with "1234") when the entire script is executed; this simple step is:

But if I (successfully) execute all the preceeding steps using each step's "Play this step only" button, and then try to execute the above step using it's "Play this step only" button I get this error:

What is the rationale for this debugging restriction when LoadRunner relies so heavily on using parameters to support persistent variables across iterations? This surely seems to defeat the TC editor's just-in-time debugging mode.

? Thanks, Terry Horwath