diff --git a/src/common/ffile.cpp b/src/common/ffile.cpp
index 0f479565d2..64dc25e3f1 100644
--- a/src/common/ffile.cpp
+++ b/src/common/ffile.cpp
@@ -157,7 +157,12 @@ bool wxFFile::Write(const wxString& s, const wxMBConv& conv)
   if ( !buf )
       return false;
 
-  const size_t size = strlen(buf); // FIXME: use buf.length() when available
+#if wxUSE_UNICODE
+  const size_t size = buf.length();
+#else
+  const size_t size = s.length();
+#endif
+
   return Write(buf, size) == size;
 }