ObjectDataSource could not find a non-generic method

I have this ASP.NET code:

<asp:DropDownList 

    ID="ddlBrokers" 

    runat="server" 

    AutoPostBack="true" 

    DataSourceID="srcBrokers" 

    DataTextField="broker" 

    DataValueField="brokerId"

/>



<asp:ObjectDataSource

    id="srcBrokers" 

    TypeName="DatabaseComponent.DBUtil" 

    SelectMethod="GetBrokers" 

    runat="server">

</asp:ObjectDataSource>

My DAL code:

public DataTable GetBrokers(bool? hasImport=null)

{

    SqlCommand cmd = new SqlCommand("usp_GetBrokers");

    if (hasImport.HasValue)

        cmd.Parameters.AddWithValue("@hasImport", hasImport);

    return FillDataTable(cmd, "brokers");

}

When the form loads I get this error:

ObjectDataSource 'srcBrokers' could not find a non-generic method 'GetBrokers' that has no parameters.

Is it my optional parameter that is causing the problem? How can I workaround this? Is it possible to have optional parameters with declarative ASP.NET code?