Index: web_ui.py
===================================================================
--- web_ui.py	(revision 7499)
+++ web_ui.py	(working copy)
@@ -1270,6 +1270,14 @@
             ticket.values.update(values)
 
         context = Context.from_request(req, ticket.resource)
+
+        # display the owner and reporter links only if the user is known
+        known_users = [user[0] for user in self.env.get_known_users()]
+        for user in 'reporter', 'owner':
+            link = '%s_link' % user
+            if ticket[user] in known_users:
+                data[link] = self._query_link(req, user, ticket[user])
+
         data.update({
             'context': context,
             'fields': fields, 'changes': changes,
@@ -1278,8 +1286,6 @@
             'action_controls': action_controls,
             'action': selected_action,
             'change_preview': change_preview,
-            'reporter_link': self._query_link(req, 'reporter', ticket['reporter']),
-            'owner_link': self._query_link(req, 'owner', ticket['owner'])
         })
 
     def rendered_changelog_entries(self, req, ticket, when=None):
Index: templates/ticket.html
===================================================================
--- templates/ticket.html	(revision 7499)
+++ templates/ticket.html	(working copy)
@@ -137,9 +137,18 @@
                                     not in ('type', 'owner')]">
             <tr>
               <th id="h_reporter">Reported by:</th>
-              <td headers="h_reporter" class="searchable">${reporter_link}</td>
+              <td headers="h_reporter" class="searchable">
+		<py:choose test="defined('reporter_link')">
+		  <py:when test="True">${reporter_link}</py:when>
+		  <py:otherwise>${authorinfo(ticket.reporter)}</py:otherwise>
+		</py:choose>
+	      </td>
               <th id="h_owner">Owned by:</th>
-              <td headers="h_owner">${owner_link}
+              <td headers="h_owner">		
+		<py:choose test="defined('owner_link')">
+		  <py:when test="True">${owner_link}</py:when>
+		  <py:otherwise>${ticket.owner and authorinfo(ticket.owner) or ''}</py:otherwise>
+		</py:choose>
               </td>
             </tr>
             <tr py:for="row in group(fields, 2, lambda f: f.type != 'textarea')"