diff --git a/eo/src/EO.h b/eo/src/EO.h
index 05e39491..fd807d0c 100644
--- a/eo/src/EO.h
+++ b/eo/src/EO.h
@@ -115,7 +115,10 @@ public:
    * @param _os A ostream.
    */
   virtual void printOn(ostream& _os) const {
-    _os << repFitness << ' '; // trailing space to make reading in that much easier
+    if (invalid())
+      _os << "INVALID ";
+    else
+      _os << repFitness << ' '; // trailing space to make reading in that much easier
   }
   
   //@}