diff --git a/src/p_lx_elf.h b/src/p_lx_elf.h
index f963d05c..a749e90a 100644
--- a/src/p_lx_elf.h
+++ b/src/p_lx_elf.h
@@ -40,6 +40,7 @@ public:
     PackLinuxElf(InputFile *f);
     virtual ~PackLinuxElf();
     /*virtual void buildLoader(const Filter *);*/
+    virtual int getVersion() const override { return 14; } // upx-3.96 cannot upack, for instance
     virtual bool canUnpackVersion(int version) const { return (version >= 11); }
     virtual int  canUnpack() { return super::canUnpack(); }
 
diff --git a/src/packer.h b/src/packer.h
index fa1a1af0..3ed72373 100644
--- a/src/packer.h
+++ b/src/packer.h
@@ -116,6 +116,8 @@ public:
     virtual ~Packer();
     virtual void assertPacker() const;
 
+    // getVersion() enables detecting forward incompatibility of unpack()
+    // by old upx when newer upx changes the format of compressed output.
     virtual int getVersion() const = 0;
     // A unique integer ID for this executable format. See conf.h.
     virtual int getFormat() const = 0;