diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index e145e72b972..242b44eafc3 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1382,6 +1382,8 @@ PostmasterMain(int argc, char *argv[])
 	 * calls fork() without an immediate exec(), both of which have undefined
 	 * behavior in a multithreaded program.  A multithreaded postmaster is the
 	 * normal case on Windows, which offers neither fork() nor sigprocmask().
+	 * Currently, macOS is the only platform having pthread_is_threaded_np(),
+	 * so we need not worry whether this HINT is appropriate elsewhere.
 	 */
 	if (pthread_is_threaded_np() != 0)
 		ereport(FATAL,
@@ -5158,15 +5160,16 @@ ExitPostmaster(int status)
 
 	/*
 	 * There is no known cause for a postmaster to become multithreaded after
-	 * startup.  Recheck to account for the possibility of unknown causes.
+	 * startup.  However, we might reach here via an error exit before
+	 * reaching the test in PostmasterMain, so provide the same hint as there.
 	 * This message uses LOG level, because an unclean shutdown at this point
 	 * would usually not look much different from a clean shutdown.
 	 */
 	if (pthread_is_threaded_np() != 0)
 		ereport(LOG,
-				(errcode(ERRCODE_INTERNAL_ERROR),
-				 errmsg_internal("postmaster became multithreaded"),
-				 errdetail("Please report this to <%s>.", PACKAGE_BUGREPORT)));
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("postmaster became multithreaded"),
+				 errhint("Set the LC_ALL environment variable to a valid locale.")));
 #endif
 
 	/* should cleanup shared memory and kill all backends */