diff --git a/builtin/grep.c b/builtin/grep.c
index 3c721b75a5..b1095362fb 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -73,14 +73,14 @@ static pthread_mutex_t grep_mutex;
 
 static inline void grep_lock(void)
 {
-	if (num_threads)
-		pthread_mutex_lock(&grep_mutex);
+	assert(num_threads);
+	pthread_mutex_lock(&grep_mutex);
 }
 
 static inline void grep_unlock(void)
 {
-	if (num_threads)
-		pthread_mutex_unlock(&grep_mutex);
+	assert(num_threads);
+	pthread_mutex_unlock(&grep_mutex);
 }
 
 /* Signalled when a new work_item is added to todo. */