diff --git a/interface/wx/strconv.h b/interface/wx/strconv.h
index 9d89c37e78..9388b49135 100644
--- a/interface/wx/strconv.h
+++ b/interface/wx/strconv.h
@@ -121,7 +121,7 @@ public:
             including the terminating @c NUL character(s).
 
         @return
-            The number of character written (or which would have been written
+            The number of characters written (or which would have been written
             if it were non-@NULL) to @a dst or @c wxCONV_FAILED on error.
     */
     virtual size_t ToWChar(wchar_t* dst, size_t dstLen, const char* src,
@@ -148,8 +148,13 @@ public:
             including the terminating @c NUL character.
 
         @return
-            The number of character written (or which would have been written
-            if it were non-@NULL) to @a dst or @c wxCONV_FAILED on error.
+            If @dst is non-@NULL, the number of characters actually written to
+            it. If @dst is @NULL, the returned value is at least equal to the
+            number of characters that would have been written out if it were
+            non-@NULL, but can be larger than it under the platforms using
+            UTF-16 as @c wchar_t encoding (this allows a useful optimization in
+            the implementation of this function for UTF-32). In any case,
+            @c wxCONV_FAILED is returned on conversion error.
     */
     virtual size_t FromWChar(char* dst, size_t dstLen, const wchar_t* src,
                              size_t srcLen = wxNO_LEN) const;
diff --git a/tests/mbconv/mbconvtest.cpp b/tests/mbconv/mbconvtest.cpp
index 10985fe3d5..69044cc057 100644
--- a/tests/mbconv/mbconvtest.cpp
+++ b/tests/mbconv/mbconvtest.cpp
@@ -1096,15 +1096,16 @@ void MBConvTestCase::TestEncoder(
     memcpy( inputCopy.data(), wideBuffer, (wideChars*sizeof(wchar_t)) );
     inputCopy.data()[wideChars] = 0;
 
-    // calculate the output size
+    // calculate the output size: notice that it can be greater than the real
+    // size as the converter is allowed to estimate the maximal size needed
+    // instead of computing it precisely
     size_t outputWritten = converter.WC2MB
         (
         0,
         (const wchar_t*)inputCopy.data(),
         0
         );
-    // make sure the correct output length was calculated
-    CPPUNIT_ASSERT_EQUAL( multiBytes, outputWritten );
+    CPPUNIT_ASSERT( outputWritten >= multiBytes );
 
     // convert the string
     size_t guardBytes = 8; // to make sure we're not overrunning the output buffer