diff --git a/fmt/format.cc b/fmt/format.cc
index cd8a244c..222cc71f 100644
--- a/fmt/format.cc
+++ b/fmt/format.cc
@@ -341,16 +341,16 @@ FMT_FUNC int internal::UTF16ToUTF8::convert(WStringRef s) {
 FMT_FUNC void WindowsError::init(
     int err_code, CStringRef format_str, args args) {
   error_code_ = err_code;
-  MemoryWriter w;
-  internal::format_windows_error(w, err_code, vformat(format_str, args));
+  internal::MemoryBuffer<char> buffer;
+  internal::format_windows_error(buffer, err_code, vformat(format_str, args));
   std::runtime_error &base = *this;
-  base = std::runtime_error(w.str());
+  base = std::runtime_error(to_string(buffer));
 }
 
 FMT_FUNC void internal::format_windows_error(
-    writer &out, int error_code, StringRef message) FMT_NOEXCEPT {
+    buffer &out, int error_code, StringRef message) FMT_NOEXCEPT {
   FMT_TRY {
-    MemoryBuffer<wchar_t, INLINE_BUFFER_SIZE> buffer;
+    MemoryBuffer<wchar_t> buffer;
     buffer.resize(INLINE_BUFFER_SIZE);
     for (;;) {
       wchar_t *system_message = &buffer[0];
@@ -361,9 +361,10 @@ FMT_FUNC void internal::format_windows_error(
       if (result != 0) {
         UTF16ToUTF8 utf8_message;
         if (utf8_message.convert(system_message) == ERROR_SUCCESS) {
-          out.write(message);
-          out.write(": ");
-          out.write(utf8_message);
+          basic_writer<char> w(out);
+          w.write(message);
+          w.write(": ");
+          w.write(utf8_message);
           return;
         }
         break;
diff --git a/test/posix-mock-test.cc b/test/posix-mock-test.cc
index 3eaca21c..a87f8633 100644
--- a/test/posix-mock-test.cc
+++ b/test/posix-mock-test.cc
@@ -279,11 +279,11 @@ TEST(FileTest, Size) {
   EXPECT_GE(f.size(), 0);
   EXPECT_EQ(content.size(), static_cast<fmt::ULongLong>(f.size()));
 #ifdef _WIN32
-  fmt::MemoryWriter message;
+  fmt::internal::MemoryBuffer<char> message;
   fmt::internal::format_windows_error(
       message, ERROR_ACCESS_DENIED, "cannot get file size");
   fstat_sim = ERROR;
-  EXPECT_THROW_MSG(f.size(), fmt::WindowsError, message.str());
+  EXPECT_THROW_MSG(f.size(), fmt::WindowsError, fmt::to_string(message));
   fstat_sim = NONE;
 #else
   f.close();
diff --git a/test/util-test.cc b/test/util-test.cc
index ab149555..370a704f 100644
--- a/test/util-test.cc
+++ b/test/util-test.cc
@@ -667,7 +667,7 @@ template <typename Converter, typename Char>
 void check_utf_conversion_error(
         const char *message,
         fmt::BasicStringRef<Char> str = fmt::BasicStringRef<Char>(0, 0)) {
-  fmt::MemoryWriter out;
+  fmt::internal::MemoryBuffer<char> out;
   fmt::internal::format_windows_error(out, ERROR_INVALID_PARAMETER, message);
   fmt::SystemError error(0, "");
   try {
@@ -676,7 +676,7 @@ void check_utf_conversion_error(
     error = e;
   }
   EXPECT_EQ(ERROR_INVALID_PARAMETER, error.error_code());
-  EXPECT_EQ(out.str(), error.what());
+  EXPECT_EQ(fmt::to_string(out), error.what());
 }
 
 TEST(UtilTest, UTF16ToUTF8Error) {
@@ -752,16 +752,17 @@ TEST(UtilTest, FormatWindowsError) {
       reinterpret_cast<LPWSTR>(&message), 0, 0);
   fmt::internal::UTF16ToUTF8 utf8_message(message);
   LocalFree(message);
-  fmt::MemoryWriter actual_message;
+  fmt::internal::MemoryBuffer<char> actual_message;
   fmt::internal::format_windows_error(
       actual_message, ERROR_FILE_EXISTS, "test");
   EXPECT_EQ(fmt::format("test: {}", utf8_message.str()),
-      actual_message.str());
+      fmt::to_string(actual_message));
   actual_message.clear();
   fmt::internal::format_windows_error(
         actual_message, ERROR_FILE_EXISTS,
         fmt::StringRef(0, std::numeric_limits<size_t>::max()));
-  EXPECT_EQ(fmt::format("error {}", ERROR_FILE_EXISTS), actual_message.str());
+  EXPECT_EQ(fmt::format("error {}", ERROR_FILE_EXISTS),
+            fmt::to_string(actual_message));
 }
 
 TEST(UtilTest, FormatLongWindowsError) {
@@ -778,11 +779,11 @@ TEST(UtilTest, FormatLongWindowsError) {
   }
   fmt::internal::UTF16ToUTF8 utf8_message(message);
   LocalFree(message);
-  fmt::MemoryWriter actual_message;
+  fmt::internal::MemoryBuffer<char> actual_message;
   fmt::internal::format_windows_error(
       actual_message, provisioning_not_allowed, "test");
   EXPECT_EQ(fmt::format("test: {}", utf8_message.str()),
-      actual_message.str());
+      fmt::to_string(actual_message));
 }
 
 TEST(UtilTest, WindowsError) {
@@ -791,11 +792,12 @@ TEST(UtilTest, WindowsError) {
 }
 
 TEST(UtilTest, ReportWindowsError) {
-  fmt::MemoryWriter out;
+  fmt::internal::MemoryBuffer<char> out;
   fmt::internal::format_windows_error(out, ERROR_FILE_EXISTS, "test error");
-  out.write('\n');
+  out.push_back('\n');
   EXPECT_WRITE(stderr,
-      fmt::report_windows_error(ERROR_FILE_EXISTS, "test error"), out.str());
+      fmt::report_windows_error(ERROR_FILE_EXISTS, "test error"),
+               fmt::to_string(out));
 }
 
 #endif  // _WIN32