object cannot be cast from DBNull to other types

Dim query = "SELECT SUM(CAST(District_max_value AS INT)) FROM [OAP].[dbo].[District_Beneficiary_max_limit] WHERE [OAP].[dbo].[District_Beneficiary_max_limit].scheme_subtype = 'OSP' AND [OAP].[dbo].[District_Beneficiary_max_limit].scheme_type= @Scheme1 AND [OAP].[dbo].[District_Beneficiary_max_limit].[Current_status]= 'Active'" If GridView3.Columns(2).HeaderText = "OSP" Then ' Build your connection ' Using con As New SqlConnection(constr) ' Build a command to execute your query ' Using com As New SqlCommand(query, con) ' Open your connection ' con.Open() ' Add your parameter ' com.Parameters.AddWithValue("@Scheme1", Scheme1.SelectedValue) ' Define your sum (from your query) ' Dim querySum = Convert.ToDouble(com.ExecuteScalar()) ...