diff --git a/builtin/grep.c b/builtin/grep.c
index b1095362fb..12e62fcbf3 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -224,7 +224,8 @@ static void start_threads(struct grep_opt *opt)
 		int err;
 		struct grep_opt *o = grep_opt_dup(opt);
 		o->output = strbuf_out;
-		o->debug = 0;
+		if (i)
+			o->debug = 0;
 		compile_grep_patterns(o);
 		err = pthread_create(&threads[i], NULL, run, o);
 
@@ -1167,8 +1168,6 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	if (!opt.fixed && opt.ignore_case)
 		opt.regflags |= REG_ICASE;
 
-	compile_grep_patterns(&opt);
-
 	/*
 	 * We have to find "--" in a separate pass, because its presence
 	 * influences how we will parse arguments that come before it.
@@ -1245,6 +1244,15 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	num_threads = 0;
 #endif
 
+	if (!num_threads)
+		/*
+		 * The compiled patterns on the main path are only
+		 * used when not using threading. Otherwise
+		 * start_threads() below calls compile_grep_patterns()
+		 * for each thread.
+		 */
+		compile_grep_patterns(&opt);
+
 #ifndef NO_PTHREADS
 	if (num_threads) {
 		if (!(opt.name_only || opt.unmatch_name_only || opt.count)