• The First() aggregate function should is not necessary; the scope should be such that there is only one value for the Gender field.

    Also, the first parameter is always the foreign key field from the primary dataset. The second parameter is always the primary key field from the secondary dataset. the third parameter is always the "lookup value" from the secondary dataset. The fourth parameter is the secondary dataset name. There is no need to specify the dataset is the first three parameters.

    Try this:

    =Lookup(Fields!Gender.Value, Fields!Gender.Value, Fields!GenderDesc.Value, "GenderDataSet")