Cannot convert 'System.Collections.Generic.List<long?>' to 'System.Collections.Generic.List<long>
<p>Using an <code>ObservableCollection</code> with a LINQ query to get an <code>int</code> from a SQL Server database table ,I have come across this error. It is suppossed to return a 4 digit integer like this <code>3125</code></p>

<blockquote>
  <p>CS1503    Argument 1: cannot convert from
  'System.Collections.Generic.List&lt;long?>' to
  'System.Collections.Generic.List&lt;long>'</p>
</blockquote>

<p>How can this be possible? It is the same type in the <code>List</code>. </p>

<p>My code,</p>

<pre><code>public static ObservableCollection&lt;long&gt; SearchNameComboStaffNo(DatabaseDataContext database, string staffNameFirst, string staffNameSecond)
        {
            return new ObservableCollection&lt;long&gt;
                   (database.Staff_Data_TBLs
                    .Where(staff =&gt; staff.Staff_Name_First == staffNameFirst &amp;&amp;                             
                     staff.Staff_Name_Second == staffNameSecond)
                    .Select(staff =&gt; staff.Staff_No).ToList());
        }
</code></pre>

<p>No matter what I try, I always get a compile error. But, how can it not be able to convert the same type?</p>