diff --git a/src/common/strconv.cpp b/src/common/strconv.cpp
index 1e40478ee8..83c08ec547 100644
--- a/src/common/strconv.cpp
+++ b/src/common/strconv.cpp
@@ -80,27 +80,27 @@ static bool NotAllNULs(const char *p, size_t n)
 
 static size_t encode_utf16(wxUint32 input, wxUint16 *output)
 {
-    if (input <= 0xffff)
+    if (wxUniChar::IsBMP(input))
     {
         if (output)
             *output = (wxUint16) input;
 
         return 1;
     }
-    else if (input >= 0x110000)
-    {
-        return wxCONV_FAILED;
-    }
-    else
+    else if (wxUniChar::IsSupplementary(input))
     {
         if (output)
         {
-            *output++ = (wxUint16) ((input >> 10) + 0xd7c0);
-            *output = (wxUint16) ((input & 0x3ff) + 0xdc00);
+            *output++ = wxUniChar::HighSurrogate(input);
+            *output = wxUniChar::LowSurrogate(input);
         }
 
         return 2;
     }
+    else
+    {
+        return wxCONV_FAILED;
+    }
 }
 
 static size_t decode_utf16(const wxUint16* input, wxUint32& output)
diff --git a/src/common/ustring.cpp b/src/common/ustring.cpp
index ab70ce5ae2..87d5158234 100644
--- a/src/common/ustring.cpp
+++ b/src/common/ustring.cpp
@@ -502,7 +502,7 @@ wxScopedU16CharBuffer wxUString::utf16_str() const
 
         // TODO: error range checks
 
-        if (code < 0x10000)
+        if (wxUniChar::IsBMP(code))
            utf16_length++;
         else
            utf16_length += 2;
@@ -520,15 +520,15 @@ wxScopedU16CharBuffer wxUString::utf16_str() const
 
         // TODO: error range checks
 
-        if (code < 0x10000)
+        if (wxUniChar::IsBMP(code))
         {
            out[0] = code;
            out++;
         }
         else
         {
-           out[0] = (code - 0x10000) / 0x400 + 0xd800;
-           out[1] = (code - 0x10000) % 0x400 + 0xdc00;
+           out[0] = wxUniChar::HighSurrogate(code);
+           out[1] = wxUniChar::LowSurrogate(code);
            out += 2;
         }
     }