diff --git a/unsupported/Eigen/SpecialFunctions b/unsupported/Eigen/SpecialFunctions
index 7c7493c56..a2ad4925e 100644
--- a/unsupported/Eigen/SpecialFunctions
+++ b/unsupported/Eigen/SpecialFunctions
@@ -10,6 +10,8 @@
 #ifndef EIGEN_SPECIALFUNCTIONS_MODULE
 #define EIGEN_SPECIALFUNCTIONS_MODULE
 
+#include <math.h>
+
 #include "../../Eigen/Core"
 
 #include "../../Eigen/src/Core/util/DisableStupidWarnings.h"