diff --git a/src/org/armedbear/lisp/JavaException.java b/src/org/armedbear/lisp/JavaException.java
--- a/src/org/armedbear/lisp/JavaException.java
+++ b/src/org/armedbear/lisp/JavaException.java
@@ -87,7 +87,7 @@
     }
 
     // ### java-exception-cause java-exception => cause
-    private static final Primitive JAVA_EXCEPTION_CAUSE =
+    protected static final Primitive JAVA_EXCEPTION_CAUSE =
         new Primitive(Symbol.JAVA_EXCEPTION_CAUSE, "java-exception",
 "Returns the cause of JAVA-EXCEPTION. (The cause is the Java Throwable\n" +
 "  object that caused JAVA-EXCEPTION to be signalled.)")