SSRS report - query parameter

Question

I am using SQL server as my data source. The dataset query is like this simplified for this question:

select * from customers where custid = @ReportParam

The wizard creates a report parameter ReportParam and also maps dataset varuable @ReportParam to report parameter ReportParam. Works fine in Report Builder, but when I run it from my app I get:

System.Data.SqlClient.SqlException (0x80131904): Must declare the scalar variable 
"@ReportParam"

Not sure why it is happening. Any idea?

Thanks