diff --git a/include/pocketpy/str.h b/include/pocketpy/str.h
index 8f78d98f..6bf518a0 100644
--- a/include/pocketpy/str.h
+++ b/include/pocketpy/str.h
@@ -73,7 +73,7 @@ struct Str{
     Str escape(bool single_quote=true) const;
     int index(const Str& sub, int start=0) const;
     Str replace(const Str& old, const Str& new_, int count=-1) const;
-    std::vector<std::string_view> split(Str sep) const;
+    std::vector<std::string_view> split(const Str& sep) const;
 
     /*************unicode*************/
     int _unicode_index_to_byte(int i) const;
diff --git a/src/str.cpp b/src/str.cpp
index 264f1f73..327be002 100644
--- a/src/str.cpp
+++ b/src/str.cpp
@@ -308,7 +308,7 @@ int utf8len(unsigned char c, bool suppress){
         return _byte_index_to_unicode(size);
     }
 
-    std::vector<std::string_view> Str::split(Str sep) const{
+    std::vector<std::string_view> Str::split(const Str& sep) const{
         std::vector<std::string_view> result;
         int start = 0;
         while(true){