Nullreference exception while reading XML tags

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C#

XML

.NET

i am trying to put August 2009 in obj.Completion date and nullreference exception is occurring what am i doing wrong

<clinical_study rank="1"> 
<completion_date type="Actual">August 2009
</completion_date>

</clinical_study>

What I have tried:

XmlNodeList completiondate = xml.SelectNodes("/clinical_study");

                foreach (XmlNode xn in completiondate)
                {
                    if (obj.completion_date == null)
                    {
                        obj.completion_date = xn["completion"].InnerText.ToString();
                    }
                    else {
                        break;
                    }
                }