In the Report Preview of the Web Report Designer, there does not seem to be a way to provide a null value to a nullable data source parameter.

For example, I have a nullable DateTime parameter. After the data source is configured the data source parameter DataType is set to String, it did not resolve to nullable DateTime. Even after I change the data source parameter to DataType of DateTime it cannot accept a null value from a DateTime report parameter, it seems to be interpreted as a string instead of null.

In the Standalone Report Designer there is no problem having a DateTime report parameter passing null to a nullable DateTime data source parameter; after reconfiguring the data source in the Standalone Report Designer the DataType for the data source parameter is <DataType>System.Nullable`1[System.DateTime]</DataType> instead of <DataType>System.DateTime</DataType> as from the Web Report Designer.