User options in BAQ RDD SSRS report - GetCriteriaPromptValue gives error

I have designed a SSRS report based on BAQ and RDD. The report runs fine. Now, I want to use the Options and get the value and print it on the report. I have made a text box on the Report Header and set is value using the Code and custom function as

=Code.GetCriteriaPromptValue(First(Fields!UserCriteria.Value, “RptParameter”) , “PartDetails”).

However, I am getting the following error

“The Value expression for the textrun ‘Textbox22.Paragraphs[0].TextRuns[0]’ has a scope parameter that is not valid for an aggregate function. The scope parameter must be set to a string constant that is equal to either the name of a containing group, the name of a containing data region, or the name of a dataset”

The option values exist in the field UserCriteria and I have confirmed that. Please see below.

3
false
false
PartDetails
bit
Part Details

I need help in fixing the above error.
Thanks in advance for your support