Object can not be cast from DBNull to other types

I'm having a problem in the return "Object can not be cast from DBNull to other types" , how to proceed?

        Public Function RetornaValorTotalPremiacao(ByVal strCodigoProposta As String) As Decimal
        Try
            Dim objApoioConexao As New ApoioConexao

            objApoioConexao.AdicionaParametro(strCodigoProposta, "@CD_PROP")

            Return Convert.ToDecimal(objDMConexao.ExecutaComando(My.Settings.ConexaoAtiva, "[dbo].[usp_sel_valor_total_premiacao]", CommandType.StoredProcedure, objApoioConexao.RetornaArrayParametros))

        Catch ex As Exception
            Throw
        End Try
    End Function