diff --git a/generator/osm_o5m_source.hpp b/generator/osm_o5m_source.hpp
index 8115070305..de57d64767 100644
--- a/generator/osm_o5m_source.hpp
+++ b/generator/osm_o5m_source.hpp
@@ -354,7 +354,7 @@ public:
         break;
       }
       default:
-        CHECK(false, (""));
+        CHECK(false, ("Unexpected relation type:", kv.key));
     }
     if (ref)
     {