DBMS_LOB.substr returns null
<p><code>v_tmp_clob := DBMS_LOB.substr(v_qry_strng, dbms_lob.getlength(v_qry_strng)-10,1);</code></p>

<p>the previous line returns null though <code>v_qry_strng</code> is a CLOB of length 32897 characters ... can any one explain ?</p>