Reporting Services Parameter([rsFieldReference] The Value Expression For The...)

@StartDate is a report parameter and CustomerNo is the field I want to render on the report,I want to combine the sql ,but it display the error message below when I preview the report.If I query in the design form and input the value of the parameter ,it run correctly .
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Data sql:
declare @sql nvarchar(4000)
declare @sqlWhere nvarchar(4000)
set @sql='select CustomerNo from table1 '
set @sqlWhere=''

if @StartDate<>''
begin
set @sqlWhere =@sqlWhere + ' StartDate=''' +@StartDate + ''''
end
if ltrim(rtrim(@sqlWhere))<>''
begin
set @sqlWhere =' where ' + @sqlWhere
end
set @sql=@sql+@sqlWhere

exec sp_executeSQL @sql
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Error message:
[rsFieldReference] The Value expression for the textbox €˜CustomerNo€™ refers to the field €˜CustomerNo€™. Report item expressions can only refer to fields within the current data set scope or, if inside an aggregate, the specified data set scope.