diff --git a/libcxx/include/ostream b/libcxx/include/ostream
index 2627da2e1961..6730cabe6365 100644
--- a/libcxx/include/ostream
+++ b/libcxx/include/ostream
@@ -1276,7 +1276,7 @@ operator<<(basic_ostream<_CharT, _Traits>& __os, shared_ptr<_Y> const& __p)
 
 template <class _CharT, class _Traits, size_t _Size>
 basic_ostream<_CharT, _Traits>&
-operator<<(basic_ostream<_CharT, _Traits>& __os, bitset<_Size>& __x)
+operator<<(basic_ostream<_CharT, _Traits>& __os, const bitset<_Size>& __x)
 {
     return __os << __x.template to_string<_CharT, _Traits>
                         (use_facet<ctype<_CharT> >(__os.getloc()).widen('0'),