diff --git a/format.cc b/format.cc
index ddfa4858..df6efa66 100644
--- a/format.cc
+++ b/format.cc
@@ -612,13 +612,13 @@ FMT_FUNC void fmt::internal::report_unknown_type(char code, const char *type) {
 
 FMT_FUNC fmt::internal::UTF8ToUTF16::UTF8ToUTF16(fmt::StringRef s) {
   int length = MultiByteToWideChar(
-      CP_UTF8, MB_ERR_INVALID_CHARS, s.c_str(), -1, 0, 0);
+      CP_UTF8, MB_ERR_INVALID_CHARS, s.data(), s.size(), 0, 0);
   static const char ERROR_MSG[] = "cannot convert string from UTF-8 to UTF-16";
   if (length == 0)
     FMT_THROW(WindowsError(GetLastError(), ERROR_MSG));
   buffer_.resize(length);
   length = MultiByteToWideChar(
-    CP_UTF8, MB_ERR_INVALID_CHARS, s.c_str(), -1, &buffer_[0], length);
+    CP_UTF8, MB_ERR_INVALID_CHARS, s.data(), s.size(), &buffer_[0], length);
   if (length == 0)
     FMT_THROW(WindowsError(GetLastError(), ERROR_MSG));
 }
@@ -631,19 +631,19 @@ FMT_FUNC fmt::internal::UTF16ToUTF8::UTF16ToUTF8(fmt::WStringRef s) {
 }
 
 FMT_FUNC int fmt::internal::UTF16ToUTF8::convert(fmt::WStringRef s) {
-  int length = WideCharToMultiByte(CP_UTF8, 0, s.c_str(), -1, 0, 0, 0, 0);
+  int length = WideCharToMultiByte(CP_UTF8, 0, s.data(), s.size(), 0, 0, 0, 0);
   if (length == 0)
     return GetLastError();
   buffer_.resize(length);
   length = WideCharToMultiByte(
-    CP_UTF8, 0, s.c_str(), -1, &buffer_[0], length, 0, 0);
+    CP_UTF8, 0, s.data(), s.size(), &buffer_[0], length, 0, 0);
   if (length == 0)
     return GetLastError();
   return 0;
 }
 
 FMT_FUNC void fmt::WindowsError::init(
-    int err_code, StringRef format_str, ArgList args) {
+    int err_code, CStringRef format_str, ArgList args) {
   error_code_ = err_code;
   MemoryWriter w;
   internal::format_windows_error(w, err_code, format(format_str, args));
diff --git a/format.h b/format.h
index 2156b3d3..dfe95a77 100644
--- a/format.h
+++ b/format.h
@@ -2557,7 +2557,7 @@ void report_system_error(int error_code, StringRef message) FMT_NOEXCEPT;
 /** A Windows error. */
 class WindowsError : public SystemError {
  private:
-  void init(int error_code, StringRef format_str, ArgList args);
+  void init(int error_code, CStringRef format_str, ArgList args);
 
  public:
   /**
@@ -2588,10 +2588,10 @@ class WindowsError : public SystemError {
      }
    \endrst
   */
-  WindowsError(int error_code, StringRef message) {
+  WindowsError(int error_code, CStringRef message) {
     init(error_code, message, ArgList());
   }
-  FMT_VARIADIC_CTOR(WindowsError, init, int, StringRef)
+  FMT_VARIADIC_CTOR(WindowsError, init, int, CStringRef)
 };
 
 // Reports a Windows error without throwing an exception.