diff --git a/include/boost/program_options/detail/utf8_codecvt_facet.hpp b/include/boost/program_options/detail/utf8_codecvt_facet.hpp
index 356a5f5..59fe55c 100644
--- a/include/boost/program_options/detail/utf8_codecvt_facet.hpp
+++ b/include/boost/program_options/detail/utf8_codecvt_facet.hpp
@@ -142,7 +142,7 @@ protected:
     // How many char objects can I process to get <= max_limit
     // wchar_t objects?
     virtual int do_length(
-        const mbstate_t &,
+        mbstate_t &,
         const char * from,
         const char * from_end, 
         std::size_t max_limit
diff --git a/src/utf8_codecvt_facet.cpp b/src/utf8_codecvt_facet.cpp
index a040f16..075a3d3 100644
--- a/src/utf8_codecvt_facet.cpp
+++ b/src/utf8_codecvt_facet.cpp
@@ -157,7 +157,7 @@ std::codecvt_base::result utf8_codecvt_facet_wchar_t::do_out(
 // How many char objects can I process to get <= max_limit
 // wchar_t objects?
 int utf8_codecvt_facet_wchar_t::do_length(
-    const mbstate_t &,
+    mbstate_t &,
     const char * from,
     const char * from_end, 
     std::size_t max_limit