diff --git a/test/test_arithmetic.hpp b/test/test_arithmetic.hpp
index a74920fc..e9c8ab7a 100644
--- a/test/test_arithmetic.hpp
+++ b/test/test_arithmetic.hpp
@@ -887,8 +887,8 @@ void test_float_funcs(const boost::mpl::true_&)
    {
       a = std::numeric_limits<Real>::quiet_NaN();
       b = modf(a, &c);
-      BOOST_CHECK((isnan)(b));
-      BOOST_CHECK((isnan)(c));
+      BOOST_CHECK((boost::math::isnan)(b));
+      BOOST_CHECK((boost::math::isnan)(c));
    }
 
    a = 4;