diff --git a/builtin/grep.c b/builtin/grep.c
index 12e62fcbf3..bd008cb100 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -1238,6 +1238,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		num_threads = GREP_NUM_THREADS_DEFAULT;
 	else if (num_threads < 0)
 		die(_("invalid number of threads specified (%d)"), num_threads);
+	if (num_threads == 1)
+		num_threads = 0;
 #else
 	if (num_threads)
 		warning(_("no threads support, ignoring --threads"));