"Mandatory filed is not set" error occurred before user get into the step

Hi,

Post this problem to see if anyone can help.

I'm using vRO 5.5. I create a workflow and add three steps for user to input.

In Step1, user will select param_1. In Step2, param_2 is set as Mandatory Input and Predefined list of elements. It'll call an Action to get this list. param_1 will be the parameter of this action. See picture below:

Now I start this workflow, after I select param_1, the Action in Step2 will be called immediately and "Mandatory field is not set" error appeared in Step2. But I don't even click Next to Step2.

I don't think it's a good user expeirence. Does anyone have a solution for this issue?

Thanks,

Vicky