diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/InvalidMappingException.java b/pkl-config-java/src/main/java/org/pkl/config/java/InvalidMappingException.java
index 27070187..826b364e 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/InvalidMappingException.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/InvalidMappingException.java
@@ -24,9 +24,9 @@ import java.lang.reflect.Type;
  * <p>When this happens, the most likely explanation is that the generated code is not up-to-date.
  */
 public class InvalidMappingException extends RuntimeException {
-  String pklName;
+  private final String pklName;
 
-  String javaName;
+  private final String javaName;
 
   public InvalidMappingException(String pklName, String javaName, Exception cause) {
     super(cause);