XVM-01123 on xml parsing

Hi,

I have a problem with SQL below

declare 

 vResult xmltype;

 vSourceXML xmltype := xmltype('<DocumentList>

                 <Document>

                  <RegionISN>3783</RegionISN>

                  <ObjRef>1</ObjRef>

                 </Document>

                 <Document>

                 <ObjRef/>

                 </Document>

                </DocumentList>');

begin

  select xmlquery('/DocumentList/Document[ObjRef=1]/RegionISN' passing vSourceXML returning content) 

into vResult

from dual;  

end;

This script get XVM-01123 [FORG0001] Invalid value for cast/constructor

The problem causes empty ObjRef tag. Is this a known issue?

I know solution - /DocumentList/Document[ObjRef="1"]/RegionISN but as i see it first variant should run too and deprecated ExtractValue function works fine in my case.

Oracle Database 12c Enterprise Edition Release 12.2.0.1.0