diff --git a/include/rabit/engine.h b/include/rabit/engine.h
index 272bbb8ef..360334808 100644
--- a/include/rabit/engine.h
+++ b/include/rabit/engine.h
@@ -21,7 +21,7 @@ namespace engine {
 /*! \brief interface of core Allreduce engine */
 class IEngine {
  public:
-  /*! 
+  /*!
    * \brief Preprocessing function, that is called before AllReduce,
    *        used to prepare the data used by AllReduce
    * \param arg additional possible argument used to invoke the preprocessor
@@ -41,6 +41,8 @@ class IEngine {
   typedef void (ReduceFunction) (const void *src,
                                  void *dst, int count,
                                  const MPI::Datatype &dtype);
+  /*! \brief virtual destructor */
+  virtual ~IEngine() {}
   /*!
    * \brief performs in-place Allreduce, on sendrecvbuf
    *        this function is NOT thread-safe
@@ -83,14 +85,14 @@ class IEngine {
    * \return the version number of the model loaded
    *     if returned version == 0, this means no model has been CheckPointed
    *     the p_model is not touched, users should do necessary initialization by themselves
-   *   
+   *
    *   Common usage example:
    *      int iter = rabit::LoadCheckPoint(&model);
    *      if (iter == 0) model.InitParameters();
    *      for (i = iter; i < max_iter; ++i) {
    *        do many things, include allreduce
    *        rabit::CheckPoint(model);
-   *      } 
+   *      }
    *
    * \sa CheckPoint, VersionNumber
    */
@@ -99,7 +101,7 @@ class IEngine {
   /*!
    * \brief checkpoints the model, meaning a stage of execution was finished
    *  every time we call check point, a version number increases by ones
-   * 
+   *
    * \param global_model pointer to the globally shared model/state
    *   when calling this function, the caller needs to guarantee that the global_model
    *   is the same in every node
@@ -117,16 +119,16 @@ class IEngine {
   /*!
    * \brief This function can be used to replace CheckPoint for global_model only,
    *   when certain condition is met (see detailed explanation).
-   * 
+   *
    *   This is a "lazy" checkpoint such that only the pointer to global_model is
    *   remembered and no memory copy is taken. To use this function, the user MUST ensure that:
    *   The global_model must remain unchanged until the last call of Allreduce/Broadcast in the current version finishes.
-   *   In other words, global_model can be changed only between the last call of 
+   *   In other words, global_model can be changed only between the last call of
    *   Allreduce/Broadcast and LazyCheckPoint in the current version
-   *   
+   *
    *   For example, suppose the calling sequence is:
    *   LazyCheckPoint, code1, Allreduce, code2, Broadcast, code3, LazyCheckPoint
-   *   
+   *
    *   If the user can only change global_model in code3, then LazyCheckPoint can be used to
    *   improve the efficiency of the program.
    * \param global_model pointer to the globally shared model/state
@@ -189,14 +191,14 @@ enum DataType {
 };
 }  // namespace mpi
 /*!
- * \brief perform in-place Allreduce, on sendrecvbuf 
+ * \brief perform in-place Allreduce, on sendrecvbuf
  *   this is an internal function used by rabit to be able to compile with MPI
  *   do not use this function directly
  * \param sendrecvbuf buffer for both sending and receiving data
  * \param type_nbytes the number of bytes the type has
  * \param count number of elements to be reduced
  * \param reducer reduce function
- * \param dtype the data type 
+ * \param dtype the data type
  * \param op the reduce operator type
  * \param prepare_func Lazy preprocessing function, lazy prepare_fun(prepare_arg)
  *                     will be called by the function before performing Allreduce, to initialize the data in sendrecvbuf_.
@@ -229,7 +231,7 @@ class ReduceHandle {
    */
   void Init(IEngine::ReduceFunction redfunc, size_t type_nbytes);
   /*!
-   * \brief customized in-place all reduce operation 
+   * \brief customized in-place all reduce operation
    * \param sendrecvbuf the in place send-recv buffer
    * \param type_n4bytes size of the type, in terms of 4bytes
    * \param count number of elements to send
diff --git a/src/allreduce_robust.cc b/src/allreduce_robust.cc
index 175751842..8137c955a 100644
--- a/src/allreduce_robust.cc
+++ b/src/allreduce_robust.cc
@@ -79,7 +79,7 @@ void AllreduceRobust::Allreduce(void *sendrecvbuf_,
                                 PreprocFunction prepare_fun,
                                 void *prepare_arg) {
   // skip action in single node
-  if (world_size == 1) {
+  if (world_size == 1 || world_size == -1) {
     if (prepare_fun != NULL) prepare_fun(prepare_arg);
     return;
   }