diff --git a/toonz/sources/common/tapptools/tcolorutils.cpp b/toonz/sources/common/tapptools/tcolorutils.cpp
index d05d375..95f375d 100644
--- a/toonz/sources/common/tapptools/tcolorutils.cpp
+++ b/toonz/sources/common/tapptools/tcolorutils.cpp
@@ -13,7 +13,6 @@ 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..3c17f21 100644
--- a/toonz/sources/stdfx/pins.cpp
+++ b/toonz/sources/stdfx/pins.cpp
@@ -10,7 +10,6 @@
 #ifdef _WIN32
 #define ISNAN _isnan
 #else
-extern "C" int isnan(double);
 #define ISNAN isnan
 #endif