diff --git a/server/AyaNova/biz/SqlSelectBuilder.cs b/server/AyaNova/biz/SqlSelectBuilder.cs
index 6fc76e13..97bbbd1e 100644
--- a/server/AyaNova/biz/SqlSelectBuilder.cs
+++ b/server/AyaNova/biz/SqlSelectBuilder.cs
@@ -36,8 +36,12 @@ namespace AyaNova.Biz
             StringBuilder sb = new StringBuilder();
             sb.Append("SELECT ");
 
-            //Default ID column
+            //Default ID column for each row (always is _df_)
             ObjectField def = objectFieldsList.FirstOrDefault(x => x.Key == "_df_");
+            if (def == null)
+            {
+                throw new System.ArgumentNullException($"SqlSelectBuilder: objectFieldList for key \"{objectKey}\" is missing the _df_ default field");
+            }
             sb.Append(def.SqlIdColumn);
             sb.Append(" AS _df_ ");