return null when get value from XML in SQL : <h1>return null , when use variable for get value in XML</h1>

<pre><code>   DECLARE @source nvarchar(2000);
    set @source = 'MinExpected'
    select MinimumExpectedProperties.value('(DepnaRoot[.= 
    sql:variable("@source")]/Type)[1]','int') from tbl_AppWFTiming
</code></pre>
