I made a goof ball mistake today.  I was using LINQ to query a List<> and return the ToString() of the result.  As usual I was using FirstOrDefault() and I simply added .ToString() to the end of it.

fieldValue = (from d in customFields where d.CustomFieldId == fieldId select d.data).FirstOrDefault().ToString();

Of course things worked swimmingly until the Default (null) was hit. At that point I got an Object not set to an instance error.  I had to laugh at myself.

var fieldData = (from d in customFields where d.CustomFieldId == fieldId select d.data).FirstOrDefault();
fieldValue = fieldData != null ? fieldData.ToString() : "";