Stack trace displayed when accessing a sub-wiki of a path based wiki farm

In a wiki farm (XEM) configured to use URL path (xwiki.virtual.usepath=1), if you access a subwiki named "welcome" (for example) using :
http://<server>/xwiki/wiki/welcome
you obtain a page with the following message :
Error number 4001 in 4: Error while parsing velocity page /templates/xwikivars.vm Wrapped Exception: Failed to evaluate content with id [/templates/xwikivars.vm]

This message is followed by a "Note page" with a stack trace :
java.lang.StringIndexOutOfBoundsException: String index out of range: -1
at java.lang.String.substring(Unknown Source)
at java.lang.String.substring(Unknown Source)
at com.xpn.xwiki.XWiki.getDocumentReference(XWiki.java:5084)
at com.xpn.xwiki.XWiki.prepareDocuments(XWiki.java:5143)
at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:189)
at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:115)
....

If you use http://<server>/xwiki/wiki/welcome/ (with a final slash), all is right. The main page of the welcome wiki is displayed.