diff --git a/src/java/org/apache/poi/ss/usermodel/FormulaError.java b/src/java/org/apache/poi/ss/usermodel/FormulaError.java
index 6415fc0364..583ca06d9c 100644
--- a/src/java/org/apache/poi/ss/usermodel/FormulaError.java
+++ b/src/java/org/apache/poi/ss/usermodel/FormulaError.java
@@ -147,9 +147,9 @@ public enum FormulaError {
         return repr;
     }
 
-    private static Map<String, FormulaError> smap = new HashMap<String, FormulaError>();
-    private static Map<Byte, FormulaError> bmap = new HashMap<Byte, FormulaError>();
-    private static Map<Integer, FormulaError> imap = new HashMap<Integer, FormulaError>();
+    private static final Map<String, FormulaError> smap = new HashMap<String, FormulaError>();
+    private static final Map<Byte, FormulaError> bmap = new HashMap<Byte, FormulaError>();
+    private static final Map<Integer, FormulaError> imap = new HashMap<Integer, FormulaError>();
     static{
         for (FormulaError error : values()) {
             bmap.put(error.getCode(), error);