diff --git a/mercurial/error.py b/mercurial/error.py
--- a/mercurial/error.py
+++ b/mercurial/error.py
@@ -215,6 +215,16 @@
 
 class ProgrammingError(Hint, RuntimeError):
     """Raised if a mercurial (core or extension) developer made a mistake"""
+
+    def __init__(self, msg, *args, **kwargs):
+        if not isinstance(msg, str):
+            # This means we're on Python 3, because we got a
+            # bytes. Turn the message back into a string since this is
+            # an internal-only error that won't be printed except in a
+            # stack traces.
+            msg = msg.decode('utf8')
+        super(ProgrammingError, self).__init__(msg, *args, **kwargs)
+
     __bytes__ = _tobytes
 
 class WdirUnsupported(Exception):