--- a/mercurial/dagop.py	Sat Jun 24 13:48:04 2017 +0900
+++ b/mercurial/dagop.py	Fri Jun 23 22:15:22 2017 -0700
@@ -32,8 +32,10 @@
         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