diff -r 6c8ba31405d9 -r 882e633ac92c mercurial/profiling.py
--- a/mercurial/profiling.py	Thu Nov 21 15:38:23 2019 -0500
+++ b/mercurial/profiling.py	Thu Nov 21 15:39:03 2019 -0500
@@ -204,7 +204,7 @@
 
         If the profiler was already started, this has no effect."""
         if not self._entered:
-            raise error.ProgrammingError()
+            raise error.ProgrammingError(b'use a context manager to start')
         if self._started:
             return
         self._started = True