diff --git a/mercurial/dagop.py b/mercurial/dagop.py
--- a/mercurial/dagop.py
+++ b/mercurial/dagop.py
@@ -32,8 +32,10 @@ def _genrevancestors(repo, revs, followf
         startdepth = 0
     if stopdepth is None:
         stopdepth = _maxlogdepth
-    if stopdepth <= 0:
+    if stopdepth == 0:
         return
+    if stopdepth < 0:
+        raise error.ProgrammingError('negative stopdepth')
 
     cl = repo.changelog