How to pass null value with Drop Down List parameter in Reporting Services

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SQL

server

service

reporting

hello,
I have a drop down list in my ssrs report
When I run the report in preview mode, that drop down list have a <Select a Value> and my assumption is since I want them to pass null, I will just leave them that way.
However, when the report is ran, I receive an error saying "Please select a value for the parameter: (my parameter)". So it forces me to select an option at which I don't want to do.
Here i want to pass null value to that parameter.