There are various circumstances that may result in a "must declare the scalar variable" error when you try to run a report.

If you know of other circumstances and the associated workarounds, please contribute this information to this article.

Must Declare Scalar Variable Error and Multi-Value Parameters

When you create a multi-value parameter for a report, you add a variable to the dataset query. The text for the query variable must include the IN operator, as shown in the following example.

WHERE
  Production.ProductInventory.ProductID IN (@ProductID)

If you don’t include the parentheses around the variable, as shown above, the report fails to render and the “must declare the scalar variable” error is displayed.