diff --git a/Internal.h b/Internal.h
index b012206..2d329f1 100644
--- a/Internal.h
+++ b/Internal.h
@@ -463,6 +463,7 @@ template <class ConflictSetImpl> struct TestDriver {
     case ConflictSet::TooOld:
       return "too old";
     }
+    abort();
   }
 
   // Call until it returns true, for "done". Check internal invariants etc