diff --git a/mercurial/dagparser.py b/mercurial/dagparser.py
--- a/mercurial/dagparser.py
+++ b/mercurial/dagparser.py
@@ -181,7 +181,7 @@ def parsedag(desc):
         else:
             return labels[ref]
 
-    chiter = (c for c in desc)
+    chiter = pycompat.iterbytestr(desc)
 
     def nextch():
         return next(chiter, '\0')
diff --git a/tests/test-doctest.py b/tests/test-doctest.py
--- a/tests/test-doctest.py
+++ b/tests/test-doctest.py
@@ -48,8 +48,7 @@ testmod('mercurial.changelog')
 testmod('mercurial.color')
 testmod('mercurial.config')
 testmod('mercurial.context')
-testmod('mercurial.dagparser', optionflags=doctest.NORMALIZE_WHITESPACE,
-        py3=False)  # py3: use of str()
+testmod('mercurial.dagparser', optionflags=doctest.NORMALIZE_WHITESPACE)
 testmod('mercurial.dispatch')
 testmod('mercurial.encoding', py3=False)  # py3: multiple encoding issues
 testmod('mercurial.formatter', py3=False)  # py3: write bytes to stdout