diff --git a/core/fpdfapi/font/cpdf_cidfont.cpp b/core/fpdfapi/font/cpdf_cidfont.cpp
index 0717795..79e5e6a 100644
--- a/core/fpdfapi/font/cpdf_cidfont.cpp
+++ b/core/fpdfapi/font/cpdf_cidfont.cpp
@@ -373,9 +373,9 @@
     return 0;
 #if defined(OS_WIN)
   uint8_t buffer[32];
-  int ret = FX_WideCharToMultiByte(kCharsetCodePages[m_pCMap->GetCoding()], 0,
-                                   &unicode, 1, reinterpret_cast<char*>(buffer),
-                                   4, nullptr, nullptr);
+  int ret =
+      FX_WideCharToMultiByte(kCharsetCodePages[m_pCMap->GetCoding()], &unicode,
+                             1, reinterpret_cast<char*>(buffer), 4);
   if (ret == 1)
     return buffer[0];
   if (ret == 2)
diff --git a/core/fxcrt/fx_codepage.cpp b/core/fxcrt/fx_codepage.cpp
index f4a22ba..e7f6eb5 100644
--- a/core/fxcrt/fx_codepage.cpp
+++ b/core/fxcrt/fx_codepage.cpp
@@ -291,16 +291,13 @@
 }
 
 int FX_WideCharToMultiByte(FX_CodePage codepage,
-                           uint32_t dwFlags,
                            const wchar_t* wstr,
                            int wlen,
                            char* buf,
-                           int buflen,
-                           const char* default_str,
-                           int* pUseDefault) {
+                           int buflen) {
 #if defined(OS_WIN)
-  return WideCharToMultiByte(static_cast<UINT>(codepage), dwFlags, wstr, wlen,
-                             buf, buflen, default_str, pUseDefault);
+  return WideCharToMultiByte(static_cast<UINT>(codepage), 0, wstr, wlen, buf,
+                             buflen, nullptr, nullptr);
 #else
   int len = 0;
   for (int i = 0; i < wlen; i++) {
diff --git a/core/fxcrt/fx_codepage.h b/core/fxcrt/fx_codepage.h
index e8f7d18..265587e 100644
--- a/core/fxcrt/fx_codepage.h
+++ b/core/fxcrt/fx_codepage.h
@@ -112,13 +112,10 @@
 FX_Charset FX_GetCharsetFromInt(int value);
 bool FX_CharSetIsCJK(FX_Charset uCharset);
 int FX_WideCharToMultiByte(FX_CodePage codepage,
-                           uint32_t dwFlags,
                            const wchar_t* wstr,
                            int wlen,
                            char* buf,
-                           int buflen,
-                           const char* default_str,
-                           int* pUseDefault);
+                           int buflen);
 int FX_MultiByteToWideChar(FX_CodePage codepage,
                            uint32_t dwFlags,
                            const char* bstr,
diff --git a/core/fxcrt/widestring.cpp b/core/fxcrt/widestring.cpp
index c3ec07f..84dfe95 100644
--- a/core/fxcrt/widestring.cpp
+++ b/core/fxcrt/widestring.cpp
@@ -658,8 +658,8 @@
 
 ByteString WideString::ToDefANSI() const {
   int src_len = GetLength();
-  int dest_len = FX_WideCharToMultiByte(FX_CodePage::kDefANSI, 0, c_str(),
-                                        src_len, nullptr, 0, nullptr, nullptr);
+  int dest_len = FX_WideCharToMultiByte(FX_CodePage::kDefANSI, c_str(), src_len,
+                                        nullptr, 0);
   if (!dest_len)
     return ByteString();
 
@@ -667,8 +667,8 @@
   {
     // Span's lifetime must end before ReleaseBuffer() below.
     pdfium::span<char> dest_buf = bstr.GetBuffer(dest_len);
-    FX_WideCharToMultiByte(FX_CodePage::kDefANSI, 0, c_str(), src_len,
-                           dest_buf.data(), dest_len, nullptr, nullptr);
+    FX_WideCharToMultiByte(FX_CodePage::kDefANSI, c_str(), src_len,
+                           dest_buf.data(), dest_len);
   }
   bstr.ReleaseBuffer(dest_len);
   return bstr;