diff --git a/include/wx/sstream.h b/include/wx/sstream.h
index 19f37c58d3..47d17565a4 100644
--- a/include/wx/sstream.h
+++ b/include/wx/sstream.h
@@ -62,16 +62,8 @@ public:
     //
     // Note that the conversion object should have the life time greater than
     // this stream.
-    wxStringOutputStream(wxString *pString = NULL,
-                         wxMBConv& conv = wxConvUTF8)
-        : m_conv(conv)
-#if wxUSE_UNICODE
-        , m_unconv(0)
-#endif // wxUSE_UNICODE
-    {
-        m_str = pString ? pString : &m_strInternal;
-        m_pos = m_str->length() / sizeof(wxChar);
-    }
+    explicit wxStringOutputStream(wxString *pString = NULL,
+                                  wxMBConv& conv = wxConvUTF8);
 
     // get the string containing current output
     const wxString& GetString() const { return *m_str; }
diff --git a/interface/wx/sstream.h b/interface/wx/sstream.h
index a1250e5ff8..e9522db36e 100644
--- a/interface/wx/sstream.h
+++ b/interface/wx/sstream.h
@@ -62,7 +62,7 @@ public:
         with default value of this argument the data written to the stream must
         be valid UTF-8, pass @c wxConvISO8859_1 to deal with arbitrary 8 bit data.
     */
-    wxStringOutputStream(wxString* pString = 0, wxMBConv& conv = wxConvUTF8);
+    explicit wxStringOutputStream(wxString* pString = NULL, wxMBConv& conv = wxConvUTF8);
 
     /**
         Returns the string containing all the data written to the stream so far.
diff --git a/src/common/sstream.cpp b/src/common/sstream.cpp
index 009e2c04b6..a67cd23152 100644
--- a/src/common/sstream.cpp
+++ b/src/common/sstream.cpp
@@ -128,6 +128,23 @@ size_t wxStringInputStream::OnSysRead(void *buffer, size_t size)
 // wxStringOutputStream implementation
 // ============================================================================
 
+wxStringOutputStream::wxStringOutputStream(wxString *pString, wxMBConv& conv)
+    : m_conv(conv)
+#if wxUSE_UNICODE
+    , m_unconv(0)
+#endif // wxUSE_UNICODE
+{
+    m_str = pString ? pString : &m_strInternal;
+
+#if wxUSE_UNICODE_WCHAR
+    m_pos = m_conv.FromWChar(NULL, 0, m_str->wc_str(), m_str->length());
+#elif wxUSE_UNICODE_UTF8
+    m_pos = m_str->utf8_length();
+#else // !wxUSE_UNICODE
+    m_pos = m_str->length();
+#endif // wxUSE_UNICODE/!wxUSE_UNICODE
+}
+
 // ----------------------------------------------------------------------------
 // seek/tell
 // ----------------------------------------------------------------------------
diff --git a/tests/streams/sstream.cpp b/tests/streams/sstream.cpp
index f421b8f4e2..6b2903b2ff 100644
--- a/tests/streams/sstream.cpp
+++ b/tests/streams/sstream.cpp
@@ -119,3 +119,32 @@ void strStream::Output_Check()
 
 // Register the stream sub suite, by using some stream helper macro.
 STREAM_TEST_SUBSUITE_NAMED_REGISTRATION(strStream)
+
+TEST_CASE("wxStringOutputStream::Tell", "[stream]")
+{
+    wxStringOutputStream ss;
+    CHECK( ss.TellO() == 0 );
+
+    const char* const s = "Hello world";
+    const wxFileOffset len = strlen(s);
+
+    ss.Write(s, len);
+    CHECK( ss.TellO() == len );
+
+    wxString str(s);
+    CHECK( wxStringOutputStream(&str).TellO() == len );
+
+
+    wxMBConvUTF16 convUTF16;
+    wxStringOutputStream ss16(NULL, convUTF16);
+    CHECK( ss16.TellO() == 0 );
+
+    const wxCharBuffer s16 = convUTF16.cWC2MB(wxWCharBuffer(str.wc_str()));
+    ss16.Write(s16, s16.length());
+    CHECK( ss16.TellO() == 2*len );
+    CHECK( wxStringOutputStream(&str, convUTF16).TellO() == 2*len );
+
+    // The U+2070D character is represented by a surrogate pair in UTF-16.
+    wxString u2070D = wxString::FromUTF8("\xF0\xA0\x9C\x8D");
+    CHECK( wxStringOutputStream(&u2070D, convUTF16).TellO() == 4 );
+}