diff --git a/simgear/math/SGGeod.hxx b/simgear/math/SGGeod.hxx
index 54b2f37f..2ab2b107 100644
--- a/simgear/math/SGGeod.hxx
+++ b/simgear/math/SGGeod.hxx
@@ -20,6 +20,10 @@
 
 #include <simgear/constants.h>
 
+#ifndef _MSC_VER
+using std::isnan;
+#endif
+
 // #define SG_GEOD_NATIVE_DEGREE
 
 /// Class representing a geodetic location
@@ -352,7 +356,7 @@ inline
 bool
 SGGeod::isValid() const
 {
-  if (std::isnan(_lon) || std::isnan(_lat)) return false;
+  if (isnan(_lon) || isnan(_lat)) return false;
 #ifdef SG_GEOD_NATIVE_DEGREE
   return (_lon >= -180.0) && (_lon <= 180.0) &&
   (_lat >= -90.0) && (_lat <= 90.0);