ssrs expressions

When I use the following expression in SSRS:

=ReportItems!SalesAmount.Value/ReportItems!BusinessTypeTotal.Value

I get the following error:

Warning1 [rsRuntimeErrorInExpression] The Value expression for the textrun ‘Textbox29.Paragraphs[0].TextRuns[0]’ contains an error: Overload resolution failed because no Public '/' can be called with these arguments:      'Public Shared Operator /(d1 As Decimal, d2 As Decimal) As Decimal':          Argument matching parameter 'd2' cannot convert from 'TextBoxImpl' to 'Decimal'.

Any ideas why?

sukai