diff --git a/format.cc b/format.cc
index b70095b7..6c0905b0 100644
--- a/format.cc
+++ b/format.cc
@@ -612,14 +612,17 @@ FMT_FUNC void fmt::internal::report_unknown_type(char code, const char *type) {
 #if FMT_USE_WINDOWS_H
 
 FMT_FUNC fmt::internal::UTF8ToUTF16::UTF8ToUTF16(fmt::StringRef s) {
-  int length = MultiByteToWideChar(
-      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 (s.size() > INT_MAX)
+    FMT_THROW(WindowsError(ERROR_INVALID_PARAMETER, ERROR_MSG));
+  int s_size = static_cast<int>(s.size());
+  int length = MultiByteToWideChar(
+      CP_UTF8, MB_ERR_INVALID_CHARS, s.data(), s_size, 0, 0);
   if (length == 0)
     FMT_THROW(WindowsError(GetLastError(), ERROR_MSG));
   buffer_.resize(length + 1);
   length = MultiByteToWideChar(
-    CP_UTF8, MB_ERR_INVALID_CHARS, s.data(), s.size(), &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));
   buffer_[length] = 0;
diff --git a/test/util-test.cc b/test/util-test.cc
index a3b17330..b92171ab 100644
--- a/test/util-test.cc
+++ b/test/util-test.cc
@@ -726,12 +726,14 @@ TEST(UtilTest, UTF8ToUTF16) {
 }
 
 template <typename Converter, typename Char>
-void check_utf_conversion_error(const char *message) {
+void check_utf_conversion_error(
+        const char *message,
+        fmt::BasicStringRef<Char> str = fmt::BasicStringRef<Char>(0, 0)) {
   fmt::MemoryWriter out;
   fmt::internal::format_windows_error(out, ERROR_INVALID_PARAMETER, message);
   fmt::SystemError error(0, "");
   try {
-    Converter(fmt::BasicStringRef<Char>(0, 0));
+    (Converter)(str);
   } catch (const fmt::SystemError &e) {
     error = e;
   }
@@ -745,14 +747,17 @@ TEST(UtilTest, UTF16ToUTF8Error) {
 }
 
 TEST(UtilTest, UTF8ToUTF16Error) {
+  const char *message = "cannot convert string from UTF-8 to UTF-16";
+  check_utf_conversion_error<fmt::internal::UTF8ToUTF16, char>(message);
   check_utf_conversion_error<fmt::internal::UTF8ToUTF16, char>(
-      "cannot convert string from UTF-8 to UTF-16");
+    message, fmt::StringRef("foo", INT_MAX + 1u));
 }
 
 TEST(UtilTest, UTF16ToUTF8Convert) {
   fmt::internal::UTF16ToUTF8 u;
   EXPECT_EQ(ERROR_INVALID_PARAMETER, u.convert(fmt::WStringRef(0, 0)));
-  EXPECT_EQ(ERROR_INVALID_PARAMETER, u.convert(fmt::WStringRef(L"foo", INT_MAX + 1u)));
+  EXPECT_EQ(ERROR_INVALID_PARAMETER,
+            u.convert(fmt::WStringRef(L"foo", INT_MAX + 1u)));
 }
 #endif  // _WIN32