diff --git a/mercurial/error.py b/mercurial/error.py
--- a/mercurial/error.py
+++ b/mercurial/error.py
@@ -36,6 +36,9 @@ class ConfigError(Exception):
 class RepoError(Exception):
     pass
 
+class RepoLookupError(RepoError):
+    pass
+
 class CapabilityError(RepoError):
     pass
 
diff --git a/mercurial/hg.py b/mercurial/hg.py
--- a/mercurial/hg.py
+++ b/mercurial/hg.py
@@ -137,10 +137,12 @@ def share(ui, source, dest=None, update=
         if update is not True:
             checkout = update
         for test in (checkout, 'default', 'tip'):
+            if test is None:
+                continue
             try:
                 uprev = r.lookup(test)
                 break
-            except LookupError:
+            except error.RepoLookupError:
                 continue
         _update(r, uprev)
 
@@ -309,10 +311,12 @@ def clone(ui, source, dest=None, pull=Fa
                 if update is not True:
                     checkout = update
                 for test in (checkout, 'default', 'tip'):
+                    if test is None:
+                        continue
                     try:
                         uprev = dest_repo.lookup(test)
                         break
-                    except:
+                    except error.RepoLookupError:
                         continue
                 _update(dest_repo, uprev)
 
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -509,7 +509,7 @@ class localrepository(repo.repository):
                 key = hex(key)
         except:
             pass
-        raise error.RepoError(_("unknown revision '%s'") % key)
+        raise error.RepoLookupError(_("unknown revision '%s'") % key)
 
     def local(self):
         return True