diff --git a/model/sync-digest.h b/model/sync-digest.h
index 528180d..15c7278 100644
--- a/model/sync-digest.h
+++ b/model/sync-digest.h
@@ -64,7 +64,7 @@
    * Side effect: finalize() will be called on `this'
    */
   std::size_t
-  getHash ();
+  getHash () const;
 
   /**
    * @brief Finalize digest. All subsequent calls to "operator <<" will fire an exception