diff --git a/src/main/java/org/apache/commons/io/input/CharacterSetFilterReader.java b/src/main/java/org/apache/commons/io/input/CharacterSetFilterReader.java
index 7a2f82a..a5cb4f8 100644
--- a/src/main/java/org/apache/commons/io/input/CharacterSetFilterReader.java
+++ b/src/main/java/org/apache/commons/io/input/CharacterSetFilterReader.java
@@ -37,7 +37,8 @@
         if (skip == null) {
             return SKIP_NONE;
         }
-        return c -> Collections.unmodifiableSet(skip).contains(Integer.valueOf(c));
+        final Set<Integer> unmodifiableSet = Collections.unmodifiableSet(skip);
+        return c -> unmodifiableSet.contains(Integer.valueOf(c));
     }
 
     /**