diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -1622,12 +1622,12 @@
             calframe = inspect.getouterframes(curframe, 2)
             frameinfo = calframe[stacklevel]
 
-            self.write_err('%s at: %s:%s (%s)\n' % (
-                msg, frameinfo.filename, frameinfo.lineno,
-                frameinfo.function))
-            self.log('develwarn', '%s at: %s:%s (%s)\n',
-                     msg, frameinfo.filename, frameinfo.lineno,
-                     frameinfo.function)
+            self.write_err('%s at: %s:%d (%s)\n' % (
+                msg, pycompat.sysbytes(frameinfo.filename),
+                frameinfo.lineno, pycompat.sysbytes(frameinfo.function)))
+            self.log('develwarn', '%s at: %s:%d (%s)\n',
+                     msg, pycompat.sysbytes(frameinfo.filename),
+                     frameinfo.lineno, pycompat.sysbytes(frameinfo.function))
             curframe = calframe = frameinfo = None  # avoid cycles
 
     def deprecwarn(self, msg, version, stacklevel=2):