diff --git a/unicode.cpp b/unicode.cpp
index fd57166d24460..7fce6fb34aaf4 100644
--- a/unicode.cpp
+++ b/unicode.cpp
@@ -1623,7 +1623,7 @@ std::string unicode_cpt_to_utf8(uint32_t cp) {
     return result;
 }
 
-std::vector<uint32_t> unicode_cpts_normalize_nfd(std::vector<uint32_t> cpts) {
+std::vector<uint32_t> unicode_cpts_normalize_nfd(const std::vector<uint32_t> & cpts) {
     std::vector<uint32_t> result;
     result.reserve(cpts.size());
     for (size_t i = 0; i < cpts.size(); ++i) {
diff --git a/unicode.h b/unicode.h
index 256187d2acc0f..6d14a5a333f57 100644
--- a/unicode.h
+++ b/unicode.h
@@ -16,7 +16,7 @@
 std::string unicode_cpt_to_utf8(uint32_t cp);
 std::vector<uint32_t> unicode_cpts_from_utf8(const std::string & utf8);
 
-std::vector<uint32_t> unicode_cpts_normalize_nfd(std::vector<uint32_t> cpts);
+std::vector<uint32_t> unicode_cpts_normalize_nfd(const std::vector<uint32_t> & cpts);
 
 int unicode_cpt_type(uint32_t cp);
 int unicode_cpt_type(const std::string & utf8);