diff --git a/Sources/Plasma/PubUtilLib/plGLight/plPerspDirSlave.cpp b/Sources/Plasma/PubUtilLib/plGLight/plPerspDirSlave.cpp
index 52476bf5..1010458f 100644
--- a/Sources/Plasma/PubUtilLib/plGLight/plPerspDirSlave.cpp
+++ b/Sources/Plasma/PubUtilLib/plGLight/plPerspDirSlave.cpp
@@ -50,7 +50,7 @@ You can contact Cyan Worlds, Inc. by email legal@cyan.com
 #include <float.h>
 #include <math.h>
 
-#ifndef isnan
+#ifdef _MSC_VER
 #define isnan _isnan
 #endif
 
diff --git a/Sources/Plasma/PubUtilLib/plGLight/plShadowSlave.cpp b/Sources/Plasma/PubUtilLib/plGLight/plShadowSlave.cpp
index f98f93d1..266b56da 100644
--- a/Sources/Plasma/PubUtilLib/plGLight/plShadowSlave.cpp
+++ b/Sources/Plasma/PubUtilLib/plGLight/plShadowSlave.cpp
@@ -48,7 +48,7 @@ You can contact Cyan Worlds, Inc. by email legal@cyan.com
 #include <float.h>
 #include <math.h>
 
-#ifndef isnan
+#ifdef _MSC_VER
 #define isnan _isnan
 #endif