diff --git a/toonz/sources/common/tapptools/tcolorutils.cpp b/toonz/sources/common/tapptools/tcolorutils.cpp
index d05d375..3c5d2dc 100644
--- a/toonz/sources/common/tapptools/tcolorutils.cpp
+++ b/toonz/sources/common/tapptools/tcolorutils.cpp
@@ -6,6 +6,7 @@
 
 #include <set>
 #include <list>
+#include <cmath>
 
 typedef float KEYER_FLOAT;
 
@@ -13,7 +14,6 @@ typedef float KEYER_FLOAT;
 #ifdef _WIN32
 #define ISNAN _isnan
 #else
-extern "C" int isnan(double);
 #define ISNAN isnan
 #endif
 
diff --git a/toonz/sources/stdfx/pins.cpp b/toonz/sources/stdfx/pins.cpp
index b4ddf97..b72016e 100644
--- a/toonz/sources/stdfx/pins.cpp
+++ b/toonz/sources/stdfx/pins.cpp
@@ -6,11 +6,12 @@
 #include "tmathutil.h"
 #include "tofflinegl.h"
 
+#include <cmath>
+
 //------------------------------------------------------------------------------
 #ifdef _WIN32
 #define ISNAN _isnan
 #else
-extern "C" int isnan(double);
 #define ISNAN isnan
 #endif