diff --git a/source/core/StarString.cpp b/source/core/StarString.cpp
index 86fea67..f76c707 100644
--- a/source/core/StarString.cpp
+++ b/source/core/StarString.cpp
@@ -1136,3 +1136,7 @@ size_t hash<StringList>::operator()(StringList const& sl) const {
 }
 
 }
+
+fmt::appender fmt::formatter<Star::String>::format(Star::String const& s, format_context& ctx) const {
+  return formatter<std::string>::format(s.utf8(), ctx);
+};
\ No newline at end of file
diff --git a/source/core/StarString.hpp b/source/core/StarString.hpp
index 9ffe536..242d54c 100644
--- a/source/core/StarString.hpp
+++ b/source/core/StarString.hpp
@@ -520,11 +520,7 @@ StringList StringList::sorted(Comparator&& comparator) const {
 }
 
 template <> struct fmt::formatter<Star::String> : formatter<std::string> {
-  auto format(Star::String const& s, format_context& ctx) const;
-};
-
-auto fmt::formatter<Star::String>::format(Star::String const& s, format_context& ctx) const {
-  return formatter<std::string>::format(s.utf8(), ctx);
+  fmt::appender format(Star::String const& s, format_context& ctx) const;
 };
 
 #endif
diff --git a/source/core/StarStringView.cpp b/source/core/StarStringView.cpp
index fa62cf5..8974e1a 100644
--- a/source/core/StarStringView.cpp
+++ b/source/core/StarStringView.cpp
@@ -425,4 +425,8 @@ std::ostream& operator<<(std::ostream& os, StringView const& s) {
   return os;
 }
 
-}
\ No newline at end of file
+}
+
+fmt::appender fmt::formatter<Star::StringView>::format(Star::StringView const& s, format_context& ctx) const {
+  return formatter<std::string_view>::format(s.utf8(), ctx);
+};
\ No newline at end of file
diff --git a/source/core/StarStringView.hpp b/source/core/StarStringView.hpp
index 9193de9..3ebc952 100644
--- a/source/core/StarStringView.hpp
+++ b/source/core/StarStringView.hpp
@@ -116,11 +116,7 @@ private:
 }
 
 template <> struct fmt::formatter<Star::StringView> : formatter<std::string_view> {
-  auto format(Star::StringView const& s, format_context& ctx) const;
-};
-
-auto fmt::formatter<Star::StringView>::format(Star::StringView const& s, format_context& ctx) const {
-  return formatter<std::string_view>::format(s.utf8(), ctx);
+  fmt::appender format(Star::StringView const& s, format_context& ctx) const;
 };
 
 #endif
\ No newline at end of file