diff --git a/istl/io.hh b/istl/io.hh
index 9e62cb1b435fd520c3621b0320e3e2a28020ad98..14db3d39132443c0b1809d67a4798ed7ccf7d2fe 100644
--- a/istl/io.hh
+++ b/istl/io.hh
@@ -158,7 +158,7 @@ namespace Dune {
     {
       s << " ";                   // space in front of each entry
       s.width(width);             // set width for each entry anew
-      s << A();                   // yeah, the number !
+      s << static_cast<const K&>(A);                   // yeah, the number !
     }
   }