diff --git a/simgear/scene/sky/cloud.cxx b/simgear/scene/sky/cloud.cxx
index e5b475ae..963c18b3 100644
--- a/simgear/scene/sky/cloud.cxx
+++ b/simgear/scene/sky/cloud.cxx
@@ -21,11 +21,12 @@
 // #include <stdio.h>
 #include <math.h>
 
-// #if defined (__APPLE__) 
-// // any C++ header file undefines isinf and isnan
-// // so this should be included before <iostream>
-// inline int (isnan)(double r) { return isnan(r); } 
-// #endif
+#if defined (__APPLE__) 
+// any C++ header file undefines isinf and isnan
+// so this should be included before <iostream>
+inline int (isinf)(double r) { return isinf(r); }
+inline int (isnan)(double r) { return isnan(r); } 
+#endif
 
 #if defined(__MINGW32__)
 #define isnan(x) _isnan(x)