diff --git a/core/fxcrt/BUILD.gn b/core/fxcrt/BUILD.gn
index 400ab35..d395e4d 100644
--- a/core/fxcrt/BUILD.gn
+++ b/core/fxcrt/BUILD.gn
@@ -142,6 +142,7 @@
     "autorestorer_unittest.cpp",
     "bytestring_unittest.cpp",
     "cfx_bitstream_unittest.cpp",
+    "cfx_seekablestreamproxy_unittest.cpp",
     "cfx_widetextbuf_unittest.cpp",
     "fx_bidi_unittest.cpp",
     "fx_coordinates_unittest.cpp",
diff --git a/core/fxcrt/cfx_seekablestreamproxy.cpp b/core/fxcrt/cfx_seekablestreamproxy.cpp
index 5884bca..c0317ca 100644
--- a/core/fxcrt/cfx_seekablestreamproxy.cpp
+++ b/core/fxcrt/cfx_seekablestreamproxy.cpp
@@ -89,32 +89,16 @@
     pDst[i - 1] = static_cast<wchar_t>(pSrc[i - 1]);
 }
 
-void SwapByteOrder(wchar_t* pStr, size_t iLength) {
-  ASSERT(pStr);
-
-  uint16_t wch;
-  if (sizeof(wchar_t) > 2) {
-    while (iLength-- > 0) {
-      wch = static_cast<uint16_t>(*pStr);
-      wch = (wch >> 8) | (wch << 8);
-      wch &= 0x00FF;
-      *pStr = wch;
-      ++pStr;
-    }
-    return;
-  }
-
+void SwapByteOrder(uint16_t* pStr, size_t iLength) {
   while (iLength-- > 0) {
-    wch = static_cast<uint16_t>(*pStr);
-    wch = (wch >> 8) | (wch << 8);
-    *pStr = wch;
-    ++pStr;
+    uint16_t wch = *pStr;
+    *pStr++ = (wch >> 8) | (wch << 8);
   }
 }
 
 }  // namespace
 
-#define BOM_MASK 0x00FFFFFF
+#define BOM_UTF8_MASK 0x00FFFFFF
 #define BOM_UTF8 0x00BFBBEF
 #define BOM_UTF16_MASK 0x0000FFFF
 #define BOM_UTF16_BE 0x0000FFFE
@@ -133,7 +117,7 @@
   uint32_t bom = 0;
   ReadData(reinterpret_cast<uint8_t*>(&bom), 3);
 
-  bom &= BOM_MASK;
+  bom &= BOM_UTF8_MASK;
   if (bom == BOM_UTF8) {
     m_wBOMLength = 3;
     m_wCodePage = FX_CODEPAGE_UTF8;
@@ -217,12 +201,11 @@
     size_t iBytes = size * 2;
     size_t iLen = ReadData(reinterpret_cast<uint8_t*>(pStr), iBytes);
     size = iLen / 2;
+    if (m_wCodePage == FX_CODEPAGE_UTF16BE)
+      SwapByteOrder(static_cast<uint16_t*>(pStr), size);
+
     if (sizeof(wchar_t) > 2 && size > 0)
       UTF16ToWChar(pStr, size);
-
-    if (m_wCodePage == FX_CODEPAGE_UTF16BE)
-      SwapByteOrder(static_cast<wchar_t*>(pStr), size);
-
   } else {
     FX_FILESIZE pos = GetPosition();
     size_t iBytes = std::min(size, static_cast<size_t>(GetSize() - pos));
diff --git a/core/fxcrt/cfx_seekablestreamproxy_unittest.cpp b/core/fxcrt/cfx_seekablestreamproxy_unittest.cpp
new file mode 100644
index 0000000..b263fa8
--- /dev/null
+++ b/core/fxcrt/cfx_seekablestreamproxy_unittest.cpp
@@ -0,0 +1,70 @@
+// Copyright 2019 PDFium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/fxcrt/cfx_seekablestreamproxy.h"
+
+#include <memory>
+#include <vector>
+
+#include "core/fxcrt/cfx_readonlymemorystream.h"
+#include "core/fxcrt/retain_ptr.h"
+#include "testing/gtest/include/gtest/gtest.h"
+#include "third_party/base/ptr_util.h"
+#include "third_party/base/span.h"
+
+TEST(SeekableStreamProxyTest, NullStream) {
+  auto proxy_stream = pdfium::MakeRetain<CFX_SeekableStreamProxy>(
+      pdfium::MakeRetain<CFX_ReadOnlyMemoryStream>(
+          pdfium::make_span<const uint8_t>(nullptr, 0)));
+
+  wchar_t buffer[16];
+  EXPECT_EQ(0u, proxy_stream->ReadBlock(buffer, FX_ArraySize(buffer)));
+}
+
+TEST(SeekableStreamProxyTest, DefaultStreamBOMNotRecognized) {
+  const char data[] = "abcd";
+  auto proxy_stream = pdfium::MakeRetain<CFX_SeekableStreamProxy>(
+      pdfium::MakeRetain<CFX_ReadOnlyMemoryStream>(pdfium::make_span(
+          reinterpret_cast<const uint8_t*>(data), sizeof(data) - 1)));
+
+  wchar_t buffer[16];
+  EXPECT_EQ(0u, proxy_stream->ReadBlock(buffer, FX_ArraySize(buffer)));
+}
+
+TEST(SeekableStreamProxyTest, UTF8Stream) {
+  const char data[] = "\xEF\xBB\xBF*\xC2\xA2*";
+  auto proxy_stream = pdfium::MakeRetain<CFX_SeekableStreamProxy>(
+      pdfium::MakeRetain<CFX_ReadOnlyMemoryStream>(pdfium::make_span(
+          reinterpret_cast<const uint8_t*>(data), sizeof(data) - 1)));
+
+  wchar_t buffer[16];
+  EXPECT_EQ(3u, proxy_stream->ReadBlock(buffer, FX_ArraySize(buffer)));
+  EXPECT_EQ(L'*', buffer[0]);
+  EXPECT_EQ(L'\u00A2', buffer[1]);
+  EXPECT_EQ(L'*', buffer[2]);
+}
+
+TEST(SeekableStreamProxyTest, UTF16LEStream) {
+  const char data[] = "\xFF\xFE\x41\x00\x42\x01";
+  auto proxy_stream = pdfium::MakeRetain<CFX_SeekableStreamProxy>(
+      pdfium::MakeRetain<CFX_ReadOnlyMemoryStream>(pdfium::make_span(
+          reinterpret_cast<const uint8_t*>(data), sizeof(data) - 1)));
+
+  wchar_t buffer[16];
+  EXPECT_EQ(2u, proxy_stream->ReadBlock(buffer, FX_ArraySize(buffer)));
+  EXPECT_EQ(L'A', buffer[0]);
+  EXPECT_EQ(L'\u0142', buffer[1]);
+}
+
+TEST(SeekableStreamProxyTest, UTF16BEStream) {
+  const char data[] = "\xFE\xFF\x00\x41\x01\x42";
+  auto proxy_stream = pdfium::MakeRetain<CFX_SeekableStreamProxy>(
+      pdfium::MakeRetain<CFX_ReadOnlyMemoryStream>(pdfium::make_span(
+          reinterpret_cast<const uint8_t*>(data), sizeof(data) - 1)));
+
+  wchar_t buffer[16];
+  EXPECT_EQ(2u, proxy_stream->ReadBlock(buffer, FX_ArraySize(buffer)));
+  EXPECT_EQ(L'A', buffer[0]);
+  EXPECT_EQ(L'\u0142', buffer[1]);
+}