diff -r f0ecf2137824 -r f30c1fab7155 hgext3rd/topic/stack.py
--- a/hgext3rd/topic/stack.py	Thu Jul 11 16:00:25 2019 -0700
+++ b/hgext3rd/topic/stack.py	Fri Jul 12 10:26:41 2019 -0700
@@ -8,6 +8,7 @@
     error,
     node,
     phases,
+    pycompat,
     obsolete,
     util,
 )
@@ -204,7 +205,7 @@
                 return 0
             except error.ManyMergeDestAbort as exc:
                 # XXX we should make it easier for upstream to provide the information
-                self.behinderror = str(exc).split('-', 1)[0].rstrip()
+                self.behinderror = pycompat.bytestr(exc).split('-', 1)[0].rstrip()
                 return -1
         return 0