C# .NET MySql - handling null fields

In my MySQL table I allow NULL values in some fields.
When I itterate through the result set I get an error whenever the reader hits such a NULL value.
Here is the code:
IDataReader reader = dbcmd.ExecuteReader();

                while (reader.Read())
                {

                    string[] catalog_info = {
                (string)reader["tab_name"], (string)reader["document_name"], (string)reader["comment_text"]};

                    dataGridViewCurrentSessionPatientDocuments.Rows.Add(catalog_info);

                }

Here is the error: "Unable to cast object of type 'System.DBNull' to type 'System.String'."
What I need is to instruct the reader to substitute an emplty string ("") for a NULL value


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