diff --git a/components/esm/variantimp.cpp b/components/esm/variantimp.cpp
index 129b6c6d0..b9cd9a853 100644
--- a/components/esm/variantimp.cpp
+++ b/components/esm/variantimp.cpp
@@ -1,6 +1,7 @@
 #include "variantimp.hpp"
 
 #include <stdexcept>
+#include <cmath>
 
 #include "esmreader.hpp"
 #include "esmwriter.hpp"
@@ -52,7 +53,7 @@ void ESM::readESMVariantValue(ESMReader& esm, Variant::Format format, VarType ty
         esm.getHNT (value, "FLTV");
 
         if (type==VT_Short)
-            if (isnan(value))
+            if (std::isnan(value))
                 out = 0;
             else
                 out = static_cast<short> (value);