Why after updating the record which has a "Text" field, it shows: "Invalid BLOB handle in record Buffer."

I am using mySQL server as database.  I display the company info which has a text field(BLob) in a form. First it's ok, but when I update or insert a record in the table and display it again. It shows:"Invalid BLOB handle in record Buffer.". Can you help me? How to solve this?

My Table: CREATE TABLE Company (
                    CompanyID INTEGER UNSIGNED NOT NULL AUTO_INCREMENT,
                    CompanyName VARCHAR(80) NULL,
                    Notes TEXT NULL,
                    PRIMARY KEY(CompanyID)
                   );

Display: CompanyQuery.SQL.text:='select * from company'
           Two components in the form, one is Edit, The other is Memo for the text field.
               CompanyNameEdit.Text:=CompanyQuery.FieldByName('CompanyName').asstring;
               NotesMemo1.Text:=CompanyQuery.FieldByName('Notes').AsString;

Update:      s:='Update Company set '
                  + 'CompanyName="'+CompanyNameEdit.Text+'",'
                  + 'Notes="'+NotesMemo1.Text+'" '
                  + 'Where CompanyID='+CompanyQuery.FieldByName('CompanyID').AsString;