diff --git a/vcsserver/remote/hg.py b/vcsserver/remote/hg.py
--- a/vcsserver/remote/hg.py
+++ b/vcsserver/remote/hg.py
@@ -29,6 +29,7 @@ from mercurial import commands
 from mercurial import unionrepo
 from mercurial import verify
 from mercurial import repair
+from mercurial.error import AmbiguousPrefixLookupError
 
 import vcsserver
 from vcsserver import exceptions
@@ -735,6 +736,10 @@ class HgRemote(RemoteBase):
                     rev = rev + -1
             try:
                 ctx = self._get_ctx(repo, rev)
+            except (AmbiguousPrefixLookupError) as e:
+                e = RepoLookupError(rev)
+                e._org_exc_tb = traceback.format_exc()
+                raise exceptions.LookupException(e)(rev)
             except (TypeError, RepoLookupError, binascii.Error) as e:
                 e._org_exc_tb = traceback.format_exc()
                 raise exceptions.LookupException(e)(rev)