diff --git a/include/boost/math/interpolators/pchip.hpp b/include/boost/math/interpolators/pchip.hpp
index 3a37ddf67..6b6db5cce 100644
--- a/include/boost/math/interpolators/pchip.hpp
+++ b/include/boost/math/interpolators/pchip.hpp
@@ -20,6 +20,7 @@ public:
           Real left_endpoint_derivative = std::numeric_limits<Real>::quiet_NaN(),
           Real right_endpoint_derivative = std::numeric_limits<Real>::quiet_NaN())
     {
+        using std::isnan;
         if (x.size() < 4)
         {
             throw std::domain_error("Must be at least four data points.");
@@ -116,4 +117,4 @@ private:
 };
 
 }
-#endif
\ No newline at end of file
+#endif