diff --git a/docs/changes.txt b/docs/changes.txt
index a7653a1844..e95fd326ab 100644
--- a/docs/changes.txt
+++ b/docs/changes.txt
@@ -211,6 +211,7 @@ wxMSW:
 - Notify shell about the changes done by wxMimeTypesManager (Maarten Bent).
 - Fix wxPrintf() and friends when using MinGW with ANSI stdio option.
 - Fix strike-through support in wxFont with GDI+ (David Vanderson).
+- Fix UTF-32 conversion for non-BMP characters (ARATA Mizuki).
 
 wxOSX/Cocoa:
 
diff --git a/src/common/strconv.cpp b/src/common/strconv.cpp
index 0390dc3795..867d663f99 100644
--- a/src/common/strconv.cpp
+++ b/src/common/strconv.cpp
@@ -1887,18 +1887,6 @@ wxMBConvUTF32straight::FromWChar(char *dst, size_t dstLen,
     if ( srcLen == wxNO_LEN )
         srcLen = wxWcslen(src) + 1;
 
-    if ( !dst )
-    {
-        // optimization: return maximal space which could be needed for this
-        // string instead of the exact amount which could be less if there are
-        // any surrogates in the input
-        //
-        // we consider that surrogates are rare enough to make it worthwhile to
-        // avoid running the loop below at the cost of slightly extra memory
-        // consumption
-        return srcLen * BYTES_PER_CHAR;
-    }
-
     wxUint32 *outBuff = reinterpret_cast<wxUint32 *>(dst);
     size_t outLen = 0;
     for ( const wchar_t * const srcEnd = src + srcLen; src < srcEnd; )
@@ -1909,10 +1897,13 @@ wxMBConvUTF32straight::FromWChar(char *dst, size_t dstLen,
 
         outLen += BYTES_PER_CHAR;
 
-        if ( outLen > dstLen )
-            return wxCONV_FAILED;
+        if ( outBuff )
+        {
+            if ( outLen > dstLen )
+                return wxCONV_FAILED;
 
-        *outBuff++ = ch;
+            *outBuff++ = ch;
+        }
     }
 
     return outLen;
@@ -1965,18 +1956,6 @@ wxMBConvUTF32swap::FromWChar(char *dst, size_t dstLen,
     if ( srcLen == wxNO_LEN )
         srcLen = wxWcslen(src) + 1;
 
-    if ( !dst )
-    {
-        // optimization: return maximal space which could be needed for this
-        // string instead of the exact amount which could be less if there are
-        // any surrogates in the input
-        //
-        // we consider that surrogates are rare enough to make it worthwhile to
-        // avoid running the loop below at the cost of slightly extra memory
-        // consumption
-        return srcLen*BYTES_PER_CHAR;
-    }
-
     wxUint32 *outBuff = reinterpret_cast<wxUint32 *>(dst);
     size_t outLen = 0;
     for ( const wchar_t * const srcEnd = src + srcLen; src < srcEnd; )
@@ -1987,10 +1966,13 @@ wxMBConvUTF32swap::FromWChar(char *dst, size_t dstLen,
 
         outLen += BYTES_PER_CHAR;
 
-        if ( outLen > dstLen )
-            return wxCONV_FAILED;
+        if ( outBuff )
+        {
+            if ( outLen > dstLen )
+                return wxCONV_FAILED;
 
-        *outBuff++ = wxUINT32_SWAP_ALWAYS(ch);
+            *outBuff++ = wxUINT32_SWAP_ALWAYS(ch);
+        }
     }
 
     return outLen;
diff --git a/tests/strings/unicode.cpp b/tests/strings/unicode.cpp
index 565bd7b22b..20751f23fe 100644
--- a/tests/strings/unicode.cpp
+++ b/tests/strings/unicode.cpp
@@ -403,6 +403,19 @@ void UnicodeTestCase::ConversionUTF16()
     wxMBConvUTF16BE().cMB2WC("\xd8\x03\xdc\x01\0" /* OLD TURKIC LETTER YENISEI A */, wxNO_LEN, &len);
     CPPUNIT_ASSERT_EQUAL( 1, len );
 #endif // UTF-32 internal representation
+
+#if SIZEOF_WCHAR_T == 2
+    // Verify that the length of UTF-32 string is correct even when converting
+    // to it from a longer UTF-16 string with surrogates.
+
+    // Construct CAT FACE U+1F431 without using \U which is not supported by
+    // ancient compilers and without using \u with surrogates which is
+    // (correctly) flagged as an error by the newer ones.
+    wchar_t ws[2];
+    ws[0] = 0xd83d;
+    ws[1] = 0xdc31;
+    CPPUNIT_ASSERT_EQUAL( 4, wxMBConvUTF32BE().FromWChar(NULL, 0, ws, 2) );
+#endif // UTF-16 internal representation
 }
 
 void UnicodeTestCase::ConversionUTF32()