diff --git a/src/fileFormats/ensight/file/ensightFile.C b/src/fileFormats/ensight/file/ensightFile.C
index a81d4e06fe..2cc9eb3d4f 100644
--- a/src/fileFormats/ensight/file/ensightFile.C
+++ b/src/fileFormats/ensight/file/ensightFile.C
@@ -353,7 +353,7 @@ void Foam::ensightFile::writeList
 {
     for (const label id : addr)
     {
-        if (idx >= field.size() || std::isnan(field[id]))
+        if (id < 0 || id >= field.size() || std::isnan(field[id]))
         {
             writeUndef();
         }