diff --git a/src/string_search.h b/src/string_search.h
index 73e90f5873f767..6e76a5b8f413ea 100644
--- a/src/string_search.h
+++ b/src/string_search.h
@@ -44,7 +44,9 @@ class Vector {
 
   // Access individual vector elements - checks bounds in debug mode.
   T& operator[](size_t index) const {
+#ifdef DEBUG
     CHECK(index < length_);
+#endif
     return start_[is_forward_ ? index : (length_ - index - 1)];
   }