diff --git a/api/utf16.hpp b/api/utf16.hpp
index 9f0100865..de3e5907d 100644
--- a/api/utf16.hpp
+++ b/api/utf16.hpp
@@ -56,7 +56,7 @@ namespace jule
     {
         std::vector<jule::I32> a(s.size());
         jule::Int n = 0;
-        for (jule::Int i = 0; i < s.size(); ++i)
+        for (decltype(s.size()) i = 0; i < s.size(); ++i)
         {
             jule::U16 r = s[i];
             if (r < jule::UTF16_SURR1 || jule::UTF16_SURR3 <= r)
@@ -84,7 +84,7 @@ namespace jule
     {
         std::vector<jule::I32> runes;
         const char *str = s.c_str();
-        for (jule::Int index = 0; index < s.length();)
+        for (decltype(s.length()) index = 0; index < s.length();)
         {
             jule::I32 rune;
             jule::Int n;
@@ -100,7 +100,7 @@ namespace jule
                                   const std::size_t len)
     {
         std::vector<jule::U16> code_page(len);
-        for (jule::Int i = 0; i < len; ++i)
+        for (std::size_t i = 0; i < len; ++i)
             code_page[i] = static_cast<jule::U16>(wstr[i]);
         return jule::runes_to_utf8(jule::utf16_decode(code_page));
     }