--- a/src/conversion/ensight/part/ensightPart.C 2021-01-07 15:37:26.650557806 +0100
+++ b/src/conversion/ensight/part/ensightPart.C 2021-01-07 15:38:04.148140312 +0100
@@ -51,7 +51,7 @@
         {
             const label id = idList[i];
 
-            if (id >= field.size() || isnan(field[id]))
+            if (id >= field.size() || std::isnan(field[id]))
             {
                 return false;
             }
--- a/src/conversion/ensight/part/ensightPartIO.C 2021-01-07 15:37:26.650557806 +0100
+++ b/src/conversion/ensight/part/ensightPartIO.C 2021-01-07 15:38:38.580675503 +0100
@@ -63,7 +63,7 @@
     {
         forAll(idList, i)
         {
-            if (idList[i] >= field.size() || isnan(field[idList[i]]))
+            if (idList[i] >= field.size() || std::isnan(field[idList[i]]))
             {
                 os.writeUndef();
             }
@@ -80,7 +80,7 @@
         // no idList => perNode
         forAll(field, i)
         {
-            if (isnan(field[i]))
+            if (std::isnan(field[i]))
             {
                 os.writeUndef();
             }