diff --git a/thrift/handler.cc b/thrift/handler.cc
index 128add2050..51f912b82b 100644
--- a/thrift/handler.cc
+++ b/thrift/handler.cc
@@ -63,7 +63,7 @@ public:
             // is fine.  Also, we don't want to change its type.
             throw;
         } catch (no_such_class& nc) {
-            throw make_exception<InvalidRequestException>("unable to find class '%s'", nc.what());
+            throw make_exception<InvalidRequestException>(nc.what());
         } catch (std::exception& e) {
             // Unexpected exception, wrap it
             throw ::apache::thrift::TException(std::string("Internal server error: ") + e.what());
diff --git a/utils/class_registrator.hh b/utils/class_registrator.hh
index d373d8fabb..31c4ed1e85 100644
--- a/utils/class_registrator.hh
+++ b/utils/class_registrator.hh
@@ -52,7 +52,7 @@ template<typename BaseType, typename... Args>
 std::unique_ptr<BaseType> class_registry<BaseType, Args...>::create(const sstring& name, Args... args) {
     auto it = _classes.find(name);
     if (it == _classes.end()) {
-        throw no_such_class(name);
+        throw no_such_class(sstring("unable to find class '") + name + sstring("'"));
     }
     return it->second(args...);
 }