diff --git a/components/fpspreadsheet/fpspreadsheet.pas b/components/fpspreadsheet/fpspreadsheet.pas
index 3c33372a8..2533a09e6 100755
--- a/components/fpspreadsheet/fpspreadsheet.pas
+++ b/components/fpspreadsheet/fpspreadsheet.pas
@@ -102,7 +102,7 @@ type
     fekIsLOGICAL, fekIsNA, fekIsNONTEXT, fekIsNUMBER, fekIsRef, fekIsTEXT,
     fekValue,
     { Other operations }
-    fekOpSUM
+    fekOpSUM {Unary sum operation. Note: CANNOT be used for summing sell contents; use fekSUM}
     );
 
   TsFormulaElement = record
diff --git a/components/fpspreadsheet/xlsbiff8.pas b/components/fpspreadsheet/xlsbiff8.pas
index ce847bc4d..12995d471 100755
--- a/components/fpspreadsheet/xlsbiff8.pas
+++ b/components/fpspreadsheet/xlsbiff8.pas
@@ -1084,11 +1084,14 @@ begin
 
     { Other operations }
     INT_EXCEL_TOKEN_TATTR: { fekOpSUM }
+    { 3.10, page 71: e.g. =SUM(1) is represented by token array
+    tInt(1),tAttrRum
+    }
     begin
-      // Uniry SUM Operation
-      AStream.WriteByte($10);
-      AStream.WriteByte(0);
-      AStream.WriteByte(0);
+      // Unary SUM Operation
+      AStream.WriteByte($10); //tAttrSum token (SUM with one parameter)
+      AStream.WriteByte(0); // not used
+      AStream.WriteByte(0); // not used
       Inc(RPNLength, 3);
     end;